import static java.lang.Math.*; import static java.util.Arrays.*; import java.io.*; import java.lang.reflect.*; import java.util.*; import java.util.concurrent.*; import java.util.function.*; public final class Main{ public static void main(String[] args) throws Exception{ MyReader in = new MyReader(System.in); MyWriter out = new MyWriter(System.out,false),log = new MyWriter(System.err,true); int T = Solver.multi ? in.it() : 1; while (T-- > 0) Optional.ofNullable(new Solver(in,out,log).solve()).ifPresent(out::println); out.flush(); } } class Solver extends BaseSolver{ public Solver(MyReader in,MyWriter out,MyWriter log){ super(in,out,log); } public static boolean multi = false; public Object solve(){ int N = in.it(); int[] A = in.it(N); int i = 1,j = A[0]; Combin cm = new Combin(); long ans = 0; while (j > 0) { long tmp = cm.nCr(i +j -2,i -1); int a = i < N && j <= A[i] && j < A[i -1] ? 0 : 1; if (i == N || A[i] < j) { if (j == A[i -1]) tmp <<= 1; j--; } else i++; ans += a *tmp; } return ans %mod; } } class Combin{ int n = 2; long[] f,fi; long mod = Util.mod; public Combin(int n){ this(); grow(n); } public Combin(){ f = fi = new long[]{1, 1}; } public void grow(int n){ n = min((int) mod,n); f = copyOf(f,n); fi = copyOf(fi,n); for (int i = this.n;i < n;i++) f[i] = f[i -1] *i %mod; fi[n -1] = pow(f[n -1],mod -2); for (int i = n;--i > this.n;) fi[i -1] = fi[i] *i %mod; this.n = n; } private long pow(long x,long n){ long ret = 1; for (x %= mod;0 < n;x = x *x %mod,n >>= 1) if ((n &1) == 1) ret = ret *x %mod; return ret; } public long nHr(int n,int r){ return r < 0 ? 0 : nCr(n +r -1,r); } public long nCr(int n,int r){ if (r < 0 || n -r < 0) return 0; if (this.n <= n) grow(max(this.n <<1,n +1)); return f[n] *(fi[r] *fi[n -r] %mod) %mod; } } class Data extends BaseV{ long v; public Data(long v){ this.v = v; } } abstract class Seg { private int n,log; private V[] val; private F[] lazy; protected Seg(int n){ this.n = n; while (1 < 0;merge(i)) (val[i] = e()).sz = val[i <<1].sz +val[i <<1 |1].sz; } public void upd(int i,F f){ prop(i +n,f); } public void upd(int l,int r,F f){ for (l += n,r += n;l < r;l >>= 1,r >>= 1) { if ((l &1) == 1) prop(l++,f); if ((r &1) == 1) prop(--r,f); } } public V get(int i){ return val[i +n]; } public V get(int l,int r){ V[] ret = Util.cast(new BaseV[]{e(), e()}); int i = 0; for (var v:getList(l,r)) { agg(ret[i],ret[i ^1],v); ret[i].sz = ret[i ^= 1].sz +v.sz; } return ret[i ^1]; } public V[] getList(int l,int r){ int sz = 0; for (int li = l += n,ri = r += n;li < ri;li = li +1 >>1,ri >>= 1) sz += (li &1) +(ri &1); V[] arr = Util.cast(Array.newInstance(e().getClass(),sz)); for (int i = 0;l < r;l >>= 1,r >>= 1) { if ((l &1) > 0) arr[i++] = val[l++]; if ((r &1) > 0) arr[--sz] = val[--r]; } return arr; } public V[] getPath(int i){ int sz = 32 -Integer.numberOfLeadingZeros(i +n); V[] arr = Util.cast(Array.newInstance(e().getClass(),sz)); for (i += n;0 < i;i >>= 1) arr[--sz] = val[i]; return arr; } protected V init(int i){ return e(); } protected abstract V e(); protected abstract void agg(V v,V a,V b); protected abstract void map(V v,F f); protected abstract F comp(F f,F g); protected void up(int l,int r){ for (l = oddPart(l +n),r = oddPart(r +n);l != r;) merge(l > r ? (l >>= 1) : (r >>= 1)); while (1 < l) merge(l >>= 1); } protected void down(int l,int r){ int i = log; for (l = oddPart(l +n),r = oddPart(r +n);i > 0;i--) { push(l >>i); push(r >>i); } } private void merge(int i){ agg(val[i],val[i <<1],val[i <<1 |1]); } private void push(int i){ if (lazy[i] != null) { prop(i <<1,lazy[i]); prop(i <<1 |1,lazy[i]); lazy[i] = null; } } private void prop(int i,F f){ map(val[i],f); if (i < n) { lazy[i] = lazy[i] == null ? f : comp(lazy[i],f); if (val[i].fail) { push(i); merge(i); } } } private int oddPart(int i){ return i /(i &-i); } } abstract class LazySegmentTree extends Seg{ public LazySegmentTree(int n){ super(n); } @Override public void upd(int i,F f){ upd(i,i +1,f); } @Override public void upd(int l,int r,F f){ down(l,r); super.upd(l,r,f); up(l,r); } @Override public V get(int i){ down(i,i +1); return super.get(i); } @Override public V get(int l,int r){ down(l,r); return super.get(l,r); } } class Edge { public int id,u,v; public L val; public Edge re; public Edge(int id,int u,int v,L val){ this.id = id; this.u = u; this.v = v; this.val = val; } } class Graph { public int n; public MyList> es; private MyList>[] go,bk; public Graph(int n,boolean dir){ this.n = n; go = Util.cast(new MyList[n]); bk = dir ? Util.cast(new MyList[n]) : go; for (int i = 0;i < n;i++) { go[i] = new MyList<>(); bk[i] = new MyList<>(); } es = new MyList<>(); } protected L inv(L l){ return l; } public void addEdge(int u,int v){ addEdge(u,v,null); } public void addEdge(int u,int v,L l){ var e = new Edge<>(es.size(),u,v,l); var re = new Edge<>(e.id,e.v,e.u,inv(e.val)); es.add(e); go[u].add(re.re = e); bk[v].add(e.re = re); } public MyList> go(int u){ return go[u]; } public MyList> bk(int u){ return bk[u]; } } abstract class AVLSegmentTree { private V e = e(); private Node root; private V[] ret = Util.cast(new BaseV[2]); private int ri; public AVLSegmentTree(int n){ this(); root = new Node(e(),n); } public AVLSegmentTree(){ ret[ri] = e(); ri = 1; } public void build(int n,IntFunction init){ root = build(0,n,init); } private Node build(int l,int r,IntFunction init){ if (r -l == 1) return new Node(init.apply(l),1); var ret = new Node(e(),r -l); ret.lft = build(l,l +r >>1,init); ret.rht = build(l +r >>1,r,init); return ret.update(); } public void add(V v){ add(v,1); } public void add(V v,int k){ ins(size(),v,k); } public void ins(int i,V v){ ins(i,v,1); } public void ins(int i,V v,int k){ root = root == null ? new Node(v,k) : ins(root,i,v,k); } private Node ins(Node nd,int i,V v,int k){ if (nd.lft == null && (i == 0 || i == nd.sz)) split(nd,i == 0 ? 1 : -1,v,k,nd.sz +k); else { if (nd.lft == null) split(nd,1,ag(e(),e,nd.val),i,nd.sz); else nd.push(); if (i < nd.lft.sz) nd.lft = ins(nd.lft,i,v,k); else nd.rht = ins(nd.rht,i -nd.lft.sz,v,k); } return balance(nd); } public void del(int i){ root = del(root,i); } private Node del(Node nd,int i){ if (nd.lft == null) return 0 < --nd.sz ? nd : null; nd.push(); int c = i < nd.lft.sz ? -1 : 1; Node del = c < 0 ? del(nd.lft,i) : del(nd.rht,i -nd.lft.sz); if (del == null) return nd.cld(-c); nd.cld(c,del); return balance(nd); } public void upd(int i,F f){ upd(i,i +1,f); } public void upd(int l,int r,F f){ if (size() < r) add(e(),r -size()); root = upd(root,l,r,f); } private Node upd(Node nd,int l,int r,F f){ if (l == 0 && r == nd.sz) nd.prop(f); else if (l < r) { if (nd.lft == null) split(nd,1,ag(e(),e,nd.val),0 < l ? l : r,nd.sz); else nd.push(); if (l < nd.lft.sz) nd.lft = upd(nd.lft,l,min(nd.lft.sz,r),f); if (nd.lft.sz < r) nd.rht = upd(nd.rht,max(0,l -nd.lft.sz),r -nd.lft.sz,f); nd = balance(nd); } return nd; } public void toggle(int l,int r){ root = l < r ? toggle(root,l,r) : root; } private Node toggle(Node nd,int l,int r){ nd.push(); if (0 < l) { split(nd,l); nd = merge(nd.lft,nd,toggle(nd.rht,0,r -l)); } else if (r < nd.sz) { split(nd,r); nd = merge(toggle(nd.lft,l,r),nd,nd.rht); } else nd.toggle(); return nd; } private void split(Node nd,int i){ if (nd.lft == null) split(nd,1,ag(e(),e,nd.val),i,nd.sz); else { nd.push(); if (i < nd.lft.sz) { split(nd.lft,i); var lft = nd.lft; nd.lft = lft.lft; nd.rht = merge(lft.rht,lft,nd.rht); } else if (nd.lft.sz < i) { split(nd.rht,i -nd.lft.sz); var rht = nd.rht; nd.rht = rht.rht; nd.lft = merge(nd.lft,rht,rht.lft); } } } private Node merge(Node lft,Node nd,Node rht){ if (abs(lft.rnk -rht.rnk) < 2) { nd.lft = lft; nd.rht = rht; } else if (lft.rnk > rht.rnk) { lft.push(); lft.rht = merge(lft.rht,nd,rht); nd = lft; } else if (lft.rnk < rht.rnk) { rht.push(); rht.lft = merge(lft,nd,rht.lft); nd = rht; } return balance(nd); } public V get(int i){ return get(root,i); } private V get(Node nd,int i){ if (nd.lft == null) return nd.val; nd.push(); return i < nd.lft.sz ? get(nd.lft,i) : get(nd.rht,i -nd.lft.sz); } public V get(int l,int r){ ret[ri] = e(); ri ^= 1; if (root != null) get(root,l,min(r,size())); return ret[ri ^= 1]; } private void get(Node nd,int l,int r){ if (0 == l && r == nd.sz) ag(ret[ri],ret[ri ^= 1],nd.val()); else if (nd.lft == null) ag(ret[ri],ret[ri ^= 1],pw(nd.val,r -l)); else { nd.push(); if (l < nd.lft.sz) get(nd.lft,l,min(nd.lft.sz,r)); if (nd.lft.sz < r) get(nd.rht,max(0,l -nd.lft.sz),r -nd.lft.sz); } } public V all(){ return root == null ? e : root.val(); } public int size(){ return root == null ? 0 : root.sz; } protected abstract V e(); protected abstract void agg(V v,V a,V b); protected abstract void map(V v,F f); protected abstract F comp(F f,F g); protected abstract void tog(V v); private V ag(V v,V a,V b){ agg(v,a,b); v.sz = a.sz +b.sz; return v; } protected V pow(V a,int n){ V ret = e(); for (V t = ag(e(),e,a);0 < n;n >>= 1,t = ag(e(),t,t)) if (0 < (n &1)) ret = ag(e(),ret,t); return ret; } private V pw(V a,int n){ V ret = pow(a,n); ret.sz = n; return ret; } private void split(Node nd,int c,V vl,int i,int sz){ nd.cld(-c,new Node(vl,i)); nd.cld(c,new Node(nd.val,sz -i)); nd.val = e(); } private Node balance(Node nd){ return (1 < abs(nd.bis = nd.rht.rnk -nd.lft.rnk) ? (nd = rotate(nd)) : nd).update(); } private Node rotate(Node u){ var v = u.cld(u.bis); v.push(); if (u.bis *v.bis < -1) v = rotate(v); u.cld(u.bis,v.cld(-u.bis)); v.cld(-u.bis,u); u.update(); return v; } private class Node{ private int sz,bis,rnk,tog; private V val; private F laz; private Node lft,rht; private Node(V val,int sz){ this.sz = sz; this.val = val; val.sz = 1; } private Node update(){ bis = rht.rnk -lft.rnk; rnk = max(lft.rnk,rht.rnk) +1; ag(val,lft.val(),rht.val()); sz = val.sz; return this; } private void push(){ if (laz != null) { lft.prop(laz); rht.prop(laz); laz = null; } if (0 < tog) { lft.toggle(); rht.toggle(); tog = 0; } } private void prop(F f){ map(val,f); if (lft != null) laz = laz == null ? f : comp(laz,f); } private Node toggle(){ bis *= -1; var tn = lft; lft = rht; rht = tn; tog(val); if (lft != null) tog ^= 1; return this; } private Node cld(int c){ return c < 0 ? lft : rht; } private void cld(int c,Node nd){ nd = c < 0 ? (lft = nd) : (rht = nd); } private V val(){ return lft == null && 1 < sz ? pw(val,sz) : val; } } } abstract class BaseV{ public int sz; public boolean fail; } class MyStack extends MyList{ public T pop(){ return remove(size() -1); } public T peek(){ return get(size() -1); } } class MyList implements Iterable{ private T[] arr; private int sz; public MyList(){ this(16); } public MyList(int n){ arr = Util.cast(new Object[n]); } public boolean isEmpty(){ return sz == 0; } public int size(){ return sz; } public T get(int i){ return arr[i]; } public void add(T t){ (arr = sz < arr.length ? arr : copyOf(arr,sz *5 >>2))[sz++] = t; } public T remove(int i){ var ret = arr[i]; sz--; for (int j = i;j < sz;j++) arr[j] = arr[j +1]; return ret; } public T removeFast(int i){ var ret = arr[i]; arr[i] = arr[--sz]; return ret; } public void sort(){ sort(Util.cast(Comparator.naturalOrder())); } public void sort(Comparator cmp){ Arrays.sort(arr,0,sz,cmp); } @Override public Iterator iterator(){ return new Iterator<>(){ int i = 0; @Override public boolean hasNext(){ return i < sz; } @Override public T next(){ return arr[i++]; } }; } public MyList map(Function func){ MyList ret = new MyList<>(sz); forEach(t -> ret.add(func.apply(t))); return ret; } public T[] toArray(){ if (sz == 0) return Util.cast(new Object[0]); T[] ret = Util.cast(Array.newInstance(arr[0].getClass(),sz)); for (int i = 0;i < sz;i++) ret[i] = arr[i]; return ret; } public void swap(int i,int j){ var t = arr[i]; arr[i] = arr[j]; arr[j] = t; } public void set(int i,T t){ arr[i] = t; } } class BaseSolver extends Util{ public MyReader in; public MyWriter out,log; public BaseSolver(MyReader in,MyWriter out,MyWriter log){ this.in = in; this.out = out; this.log = log; } protected long inv(long x){ return pow(x,mod -2); } protected long pow(long x,long n){ return pow(x,n,Util.mod); } protected long pow(long x,long n,long mod){ long ret = 1; for (x %= mod;0 < n;x = x *x %mod,n >>= 1) if ((n &1) == 1) ret = ret *x %mod; return ret; } protected int bSearchI(int o,int n,IntPredicate judge){ if (!judge.test(o)) return o -Integer.signum(n -o); for (int m = 0;1 < abs(n -o);) m = judge.test(m = o +n >>1) ? (o = m) : (n = m); return o; } protected long bSearchL(long o,long n,LongPredicate judge){ for (long m = 0;1 < abs(n -o);) m = judge.test(m = o +n >>1) ? (o = m) : (n = m); return o; } protected double bSearchD(double o,double n,DoublePredicate judge){ for (double m,c = 0;c < 100;c++) m = judge.test(m = (o +n) /2) ? (o = m) : (n = m); return o; } protected long gcd(long a,long b){ while (0 < b) { long t = a; a = b; b = t %b; } return a; } public long lcm(long a,long b){ return b /gcd(a,b) *a; } protected long ceil(long a,long b){ return (a +b -1) /b; } } class Util{ public static String yes = "Yes",no = "No"; public static int infI = (1 <<30) -1; public static long infL = (1L <<61 |1 <<30) -1, mod = 998244353; public static Random rd = ThreadLocalRandom.current(); private long st = System.currentTimeMillis(); protected long elapsed(){ return System.currentTimeMillis() -st; } protected void reset(){ st = System.currentTimeMillis(); } public static int[] arrI(int N,IntUnaryOperator f){ int[] ret = new int[N]; setAll(ret,f); return ret; } public static long[] arrL(int N,IntToLongFunction f){ long[] ret = new long[N]; setAll(ret,f); return ret; } public static double[] arrD(int N,IntToDoubleFunction f){ double[] ret = new double[N]; setAll(ret,f); return ret; } public static T[] arr(T[] arr,IntFunction f){ setAll(arr,f); return arr; } public int[][] addId(int[][] T){ return arr(new int[T.length][],i -> { int[] t = copyOf(T[i],T[i].length +1); t[t.length -1] = i; return t; }); } @SuppressWarnings("unchecked") public static T cast(Object obj){ return (T) obj; } } class MyReader{ private byte[] buf = new byte[1 <<16]; private int ptr,tail; private InputStream in; public MyReader(InputStream in){ this.in = in; } private byte read(){ if (ptr == tail) try { tail = in.read(buf); ptr = 0; } catch (IOException e) {} return buf[ptr++]; } private boolean isPrintable(byte c){ return 32 < c && c < 127; } private byte nextPrintable(){ byte ret = read(); while (!isPrintable(ret)) ret = read(); return ret; } public int it(){ return toIntExact(lg()); } public int[] it(int N){ return Util.arrI(N,i -> it()); } public int[][] it(int H,int W){ return Util.arr(new int[H][],i -> it(W)); } public int idx(){ return it() -1; } public int[] idx(int N){ return Util.arrI(N,i -> idx()); } public int[][] idx(int H,int W){ return Util.arr(new int[H][],i -> idx(W)); } public long lg(){ byte i = nextPrintable(); boolean negative = i == 45; long n = negative ? 0 : i -'0'; while (isPrintable(i = read())) n = 10 *n +i -'0'; return negative ? -n : n; } public long[] lg(int N){ return Util.arrL(N,i -> lg()); } public long[][] lg(int H,int W){ return Util.arr(new long[H][],i -> lg(W)); } public double dbl(){ return Double.parseDouble(str()); } public double[] dbl(int N){ return Util.arrD(N,i -> dbl()); } public double[][] dbl(int H,int W){ return Util.arr(new double[H][],i -> dbl(W)); } public char[] ch(){ return str().toCharArray(); } public char[][] ch(int H){ return Util.arr(new char[H][],i -> ch()); } public String line(){ StringBuilder sb = new StringBuilder(); for (byte c;(c = read()) != '\n';) sb.append((char) c); return sb.toString(); } public String str(){ StringBuilder sb = new StringBuilder(); sb.append((char) nextPrintable()); for (byte c;isPrintable(c = read());) sb.append((char) c); return sb.toString(); } public String[] str(int N){ return Util.arr(new String[N],i -> str()); } public String[][] str(int H,int W){ return Util.arr(new String[H][],i -> str(W)); } } class MyWriter{ private OutputStream out; private byte[] buf = new byte[1 <<16],ibuf = new byte[20]; private int tail; private boolean autoflush; public MyWriter(OutputStream out,boolean autoflush){ this.out = out; this.autoflush = autoflush; } public void flush(){ try { out.write(buf,0,tail); tail = 0; } catch (IOException e) { e.printStackTrace(); } } private void ln(){ write((byte) '\n'); if (autoflush) flush(); } private void write(byte b){ buf[tail++] = b; if (tail == buf.length) flush(); } private void write(long n){ if (n < 0) { n = -n; write((byte) '-'); } int i = ibuf.length; do { ibuf[--i] = (byte) (n %10 +'0'); n /= 10; } while (n > 0); while (i < ibuf.length) write(ibuf[i++]); } private void print(Object obj){ if (obj instanceof Boolean) print((boolean) obj ? Util.yes : Util.no); else if (obj instanceof Integer) write((int) obj); else if (obj instanceof Long) write((long) obj); else if (obj instanceof char[]) for (char b:(char[]) obj) write((byte) b); else if (obj.getClass().isArray()) { int l = Array.getLength(obj); for (int i = 0;i < l;i++) { print(Array.get(obj,i)); if (i +1 < l) write((byte) ' '); } } else print(Objects.toString(obj).toCharArray()); } public void println(Object obj){ if (obj == null) obj = "null"; if (obj instanceof Iterable) for (Object e:(Iterable) obj) println(e); else if (obj.getClass().isArray() && Array.getLength(obj) > 0 && Array.get(obj,0).getClass().isArray()) { int l = Array.getLength(obj); for (int i = 0;i < l;i++) println(Array.get(obj,i)); } else { print(obj); ln(); } } public void printlns(Object... o){ print(o); ln(); } }