#include <bits/stdc++.h>
#define int long long
using namespace std;
const int mod = 998244353;
int n, m, a[5005], sum, jie[30000007], inv[30000007], f[5005][5005], ssum[5005], cnt;
inline int ksm(int p, int q = mod - 2) {
	int base = 1;
	while (q) {
		if (q & 1) base = base * p % mod;
		q >>= 1;
		p = p * p % mod;
	}
	return base;
}
inline int C(int p, int q) {
	if (p < q) return 0;
	return jie[p] * inv[q] % mod * inv[p - q] % mod;
}
signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		sum += a[i];
		ssum[i] = ssum[i - 1] + a[i];
	}
	cnt = sum;
	sum += 2 * n - m;
	jie[0] = jie[1] = 1;
	for (int i = 2; i <= sum; i++) {
		jie[i] = jie[i - 1] * i % mod;
	}
	inv[sum] = ksm(jie[sum]);
	for (int i = sum - 1; i >= 0; i--) {
		inv[i] = (inv[i + 1] * (i + 1)) % mod;
	}
	f[m][0] = 1;
	for (int i = m; i <= n; i++) {
		for (int j = 0 + (i == m); j <= n; j++) {
			if (j > i) f[i][j] = 0;
			else f[i][j] = f[i][j - 1] * C(cnt - ssum[j - 1] + n - (j) + (n - i), a[j]) % mod + f[i - 1][j];
			f[i][j] %= mod;
		}
//		for (int j = 0; j <= n; j++) {
//			cout << f[i][j] << ' ';
///		}
//		cout << endl;
	}
	cout << f[n][n];
}