#pragma GCC optimize("Ofast") 
#include <bits/stdc++.h>
using namespace std;
using ll = long long int ;
using ld = long double ;
using P = pair<ll,ll>;
using Graph= vector<vector<ll>>;
struct edge{ll to ; ll cost ;} ;
using graph =vector<vector<edge>> ;
#define rep(i,n) for (ll i=0; i < (n); ++i)
#define rep2(i,n,m) for(ll i=n;i<=m;i++)
#define rep3(i,n,m) for(ll i=n;i>=m;i--)
#define pb push_back 
#define eb emplace_back 
#define ppb pop_back 
#define mpa make_pair 
#define fi first  
#define se second  
#define set20 cout<<fixed<<setprecision(20)  ;
const ll INF=1e18 ;   
inline void chmax(ll& a,ll b){a=max(a,b);}  
inline void chmin(ll& a,ll b){a=min(a,b);} 
long double pi=acos(-1) ;  
ll gcd(ll a, ll b) { return b?gcd(b,a%b):a;}  
ll lcm(ll a, ll b) { return a/gcd(a,b)*b;} 
ll dx[4] {1,0,-1,0} ;
ll dy[4] {0,1,0,-1} ;
#define debug cout<<888<<endl ;

// ミント
//int mod ;  //任意modではconst外す

const int mod =//1e9+7 ;//924844033;  
  998244353;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }
 
  // for prime mod
  mint inv() const { return pow(mod-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
//昆布

struct combination {
  vector<mint> fact, ifact;
  combination(int n):fact(n+1),ifact(n+1) {
    assert(n < mod); //任意modではここ消すcombmain内に
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n]*ifact[k]*ifact[n-k];
  }
  mint p(int n,int k){
    return fact[n]*ifact[n-k] ; //kは個数
  }
  }  co(1000005)
  ;


mint modpow(ll a,ll b){
 if(b==0) return 1 ;
 mint c= modpow(a,b/2) ;
 if(b%2==1) return c*c*a ;
 else return c*c ;
}

mint mmodpow(mint a,ll b){
  if(b==0) return 1ll ;
  mint c=mmodpow(a,(b/2)) ;
  if(b%2==1) return c*c*a ;
  else return c*c ;
}
mint komb(ll n,ll m){
  mint x=1 ;mint y=1 ;
  rep(i,m){
    x*= n-i ;
    y*= i+1 ;
  }
  return x/y ;
}
 
 map<ll,ll> factor(ll n){  //素因数とオーダーをマップで管理
  map <ll,ll> ord ;  
  for(ll i=2;i*i<=n;i++){
        if(n%i==0){
            int res=0;
            while(n%i==0){
                n/=i;
                res++;
            }
            ord[i]=res;
        }
    }
    if(n!=1) ord[n]++;
    return ord ;
 }

 

struct UnionFind {
  vector<int> d;
  UnionFind(int n=0): d(n,-1) {}
  int find(int x) {
    if (d[x] < 0) return x;
    return d[x] = find(d[x]);
  }
  bool unite(int x, int y) {
    x = find(x); y = find(y);
    if (x == y) return false;
    if (d[x] > d[y]) swap(x,y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }
  bool same(int x, int y) { return find(x) == find(y);}
  int size(int x) { return -d[find(x)];}
};

// sum(x) x以下の和
// sum(a,b) a以上b以下の和
template<typename T>
struct BIT {
  int n;
  vector<T> d;
  BIT(int n=0):n(n),d(n+1) {}
  void add(int i, T x=1) {  //x=1ならsumは個数のカウント
    for (i++; i <= n; i += i&-i) {
      d[i] += x;
    }
  }
  T sum(int i) {
    T x = 0;
    for (i++; i; i -= i&-i) {
      x += d[i];
    }
    return x;
  }
  T sum(int i,int j) {
    if(i>0) return sum(j)-sum(i-1);
    else return sum(j); } 
};

template< typename flow_t >
struct Dinic {
  const flow_t INF;

  struct edge {
    int to;
    flow_t cap;
    int rev;
    bool isrev;
    int idx;
  };

  vector< vector< edge > > graph;
  vector< int > min_cost, iter;

  Dinic(int V) : INF(numeric_limits< flow_t >::max()), graph(V) {}

  void add_edge(int from, int to, flow_t cap, int idx = -1) {
    graph[from].emplace_back((edge) {to, cap, (int) graph[to].size(), false, idx});
    graph[to].emplace_back((edge) {from, 0, (int) graph[from].size() - 1, true, idx});
  }

  bool bfs(int s, int t) {
    min_cost.assign(graph.size(), -1);
    queue< int > que;
    min_cost[s] = 0;
    que.push(s);
    while(!que.empty() && min_cost[t] == -1) {
      int p = que.front();
      que.pop();
      for(auto &e : graph[p]) {
        if(e.cap > 0 && min_cost[e.to] == -1) {
          min_cost[e.to] = min_cost[p] + 1;
          que.push(e.to);
        }
      }
    }
    return min_cost[t] != -1;
  }

  flow_t dfs(int idx, const int t, flow_t flow) {
    if(idx == t) return flow;
    for(int &i = iter[idx]; i < graph[idx].size(); i++) {
      edge &e = graph[idx][i];
      if(e.cap > 0 && min_cost[idx] < min_cost[e.to]) {
        flow_t d = dfs(e.to, t, min(flow, e.cap));
        if(d > 0) {
          e.cap -= d;
          graph[e.to][e.rev].cap += d;
          return d;
        }
      }
    }
    return 0;
  }

  flow_t max_flow(int s, int t) {
    flow_t flow = 0;
    while(bfs(s, t)) {
      iter.assign(graph.size(), 0);
      flow_t f = 0;
      while((f = dfs(s, t, INF)) > 0) flow += f;
    }
    return flow;
  }

  void output() {
    for(int i = 0; i < graph.size(); i++) {
      for(auto &e : graph[i]) {
        if(e.isrev) continue;
        auto &rev_e = graph[e.to][e.rev];
        cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/" << e.cap + rev_e.cap << ")" << endl;
      }
    }
  }
};

vector<ll> alldiv(ll n){
    vector<ll> ans;
    if(n<0) n*=-1;
    for(ll i=1;i*i<=n;++i){
      if(n%i==0){
        if(i*i==n) ans.pb(i);
        else{ans.pb(i);ans.pb(n/i);}
      }
    }
    return ans;
}



int main(){
  ios::sync_with_stdio(false) ;
  cin.tie(nullptr) ;
  
  ll n,m; cin>>n>>m;
  vector<ll> A(n);
  rep(i,n) cin>>A[i];

  ll sum=0ll;
  rep(i,n){
    if(i%2==0) sum+=A[i];
    else sum-=A[i];
  }

  if(sum%m){
    cout<<-1<<endl;
  }
  else{
    mint now=0ll;
    rep(i,n){
      now*=m-1;
      now+=A[i];
    }
    now/=m;
    now+=1ll;
    cout<<now<<endl;
  }

  return 0;
}