Codeforces Round #309 (Div. 2)

#include <cstdio>
#include <cstring>

typedef long long ll;
const int N = 2000;
const ll MOD = 1e9+7;
ll C[N][N];
int num[N];

int main() {
    C[0][0] = 1;
    for (int i = 1; i < N; i++) {
        C[i][0] = 1;
        for (int j = 1; j <= i; j++) {
            C[i][j] = C[i-1][j] + C[i-1][j-1];
            while (C[i][j] >= MOD) {
                C[i][j] -= MOD;
            }
        }
    }
    int k;
    scanf("%d", &k);
    for (int i = 1; i <= k; i++) {
        scanf("%d", &num[i]);
    }
    ll ans = 1;
    ll cnt = num[1];
    for (int i = 2; i <= k; i++) {
        ans = (ans*C[cnt+num[i]-1][num[i]-1])%MOD;
        cnt += num[i];
    }
    printf("%lld\n", ans);
    return 0;
}