Educational Codeforces Round 46 D. Yet Another Problem On a Subsequence

#include <bits/stdc++.h>
#define REP(i,a,b) for(int i=(a); i<(b); i++)
#define DEP(i,a,b) for(int i=(a); i>=(b); i--)
#define N 1010
using namespace std;
typedef long long LL;
const LL mod = 998244353;

int n;
LL a[N], dp[N];
LL C[N][N];

int main()
{
    scanf("%d", &n);
    C[0][0] = 1;
    REP(i, 1, n+1) {
        C[i][0] = C[i][i] = 1;
        REP(j, 1, i)
                C[i][j] = (C[i-1][j] + C[i-1][j-1]) % mod;
    }
    REP(i, 1, n+1) scanf("%lld", &a[i]);
    memset(dp, 0, sizeof(dp));
    dp[n+1] = 1;
    DEP(i, n, 1) {
        if (a[i] <= 0) {dp[i] = 0; continue;}
        REP(j, i+a[i]+1, n+2)
                dp[i] = (dp[i] + (C[j-i-1][a[i]] * dp[j] % mod)) % mod;
    }
    LL res = 0;
    REP(i, 1, n+1) res = (res + dp[i]) % mod;
    printf("%lld\n", res);
    return 0;
}