import java.io.PrintWriter import java.util.* import kotlin.math.* fun PrintWriter.solve() { val n = nextInt() val a = Array(n) { nextInt() } + arrayOf(0) var ans = ModInt(0) val cc = CombinationCalculator() var j = a[0] for (i in 0 until n) { while (j > a[i + 1]) { val v = cc.binom(i + j - 1L, j - 1L) if (j == a[i]) ans += v ans += v j-- } val v = cc.binom(i + j - 1L, j - 1L) if (j == a[i]) ans += v } println(ans) } // region ModInt class ModInt(x: Long) { companion object { const val MOD = 998244353L //const val MOD = 1000000007L } constructor(y: Int) : this(y.toLong()) val x = (x % MOD + MOD) % MOD operator fun plus(other: ModInt): ModInt { return ModInt(x + other.x) } operator fun minus(other: ModInt): ModInt { return ModInt(x - other.x) } operator fun times(other: ModInt): ModInt { return ModInt(x * other.x) } operator fun div(other: ModInt): ModInt { return this * other.inv() } fun pow(exp: Long): ModInt { if (exp == 0L) return ModInt(1L) var a = pow(exp shr 1) a *= a if (exp and 1L == 0L) return a return this * a } fun inv(): ModInt { return this.pow(MOD - 2) } override fun equals(other: Any?): Boolean { if (this === other) return true if (javaClass != other?.javaClass) return false other as ModInt if (x != other.x) return false return true } override fun hashCode(): Int { return x.hashCode() } override fun toString(): String { return "$x" } } class ModIntArray(n: Int) { private val a = LongArray(n) { 0L } constructor(n: Int, init: (Int) -> ModInt) : this(n) { for (i in 0 until n) { a[i] = init(i).x } } operator fun set(i: Int, x: ModInt) { a[i] = x.x } operator fun get(i: Int): ModInt { return ModInt(a[i]) } fun joinToString(s: String = ", "): String { return a.joinToString(s) } } fun Long.toModInt() = ModInt(this) fun Int.toModInt() = ModInt(this) fun binomSimple(n: Long, k: Long): ModInt { var numer = ModInt(1) var denom = ModInt(1) for (i in 0 until k) { numer *= ModInt(n - i) denom *= ModInt(k - i) } return numer / denom } // endregion class CombinationCalculator { companion object { private const val fac_size = 600000 } private val arrFac = Array(fac_size) { ModInt(1) } private val arrInvfac = Array(fac_size) { ModInt(1) } init { for (i in 1 until fac_size) { arrFac[i] = arrFac[i - 1] * ModInt(i) } arrInvfac[fac_size - 1] = arrFac[fac_size - 1].inv() for (i in fac_size - 2 downTo 1) { arrInvfac[i] = arrInvfac[i + 1] * ModInt(i + 1) } } fun fac(n: Int): ModInt { return arrFac[n] } fun fac(n: Long): ModInt { return fac(n.toInt()) } fun invfac(n: Int): ModInt { return arrInvfac[n] } fun invfac(n: Long): ModInt { return invfac(n.toInt()) } fun binom(n: Long, k: Long): ModInt { return if (k in 0..n) { arrFac[n.toInt()] * arrInvfac[k.toInt()] * arrInvfac[(n - k).toInt()] } else { ModInt(0) } } } fun main() { Thread(null, { val writer = PrintWriter(System.out, false) writer.solve() writer.flush() }, "solve", abs(1L.shl(26))) .apply { setUncaughtExceptionHandler { _, e -> e.printStackTrace(); kotlin.system.exitProcess(1) } } .apply { start() }.join() } // region Scanner private var st = StringTokenizer("") private val br = System.`in`.bufferedReader() fun next(): String { while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine()) return st.nextToken() } fun nextInt() = next().toInt() fun nextLong() = next().toLong() fun nextLine() = br.readLine() fun nextDouble() = next().toDouble() // endregion