rt，感谢各方大佬。

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
const int mod = 1e9 + 7;
long long f[110][100010];
int a[110];
int main()
{
	int n, k;
	scanf("%d%d", &n, &k);
	for (int i = 1; i <= n; i++) scanf("%d", a + i);
	f[0][0] = 1;
	for (int i = 1; i <= n; i++)
	{
		f[i][0] = 1;
		for (int j = 1; j <= k; j++)
		{
			int t = 0;
			if (j - a[i] >= 1) t = f[i - 1][j - a[i] - 1];
			f[i][j] = f[i - 1][j] - t + f[i][j - 1] + mod;
			f[i][j] %= mod;
		}
	}
	printf("%lld\n", f[n][k]);
	return 0;
}