B - Average Gym - 101161B 组合数学

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <string>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#define X first
#define Y second
#define lson T[rt].l
#define rson T[rt].r
#define clr(u,v); memset(u,v,sizeof(u));
#define in() freopen("data.txt","r",stdin);
#define out() freopen("ans","w",stdout);
#define Clear(Q); while (!Q.empty()) Q.pop();
#define pb push_back

using namespace std;
typedef long long LL;
typedef pair<int, int> pii;
const int maxn = 1e6 + 20;
LL C[14000 + 2][60 + 2];
const int MOD = 1e9 + 7;
LL calc(int n, int en, int sum) {
    if (sum < 0) return 0;
    LL all = C[sum + n - 1][n - 1];
    for (int i = 1; i <= n; ++i) {
        int fuck = sum - i * (en + 1) + n - 1;
        if (fuck < 0) break; //notice
        if (i & 1) {
            all = (all + MOD - C[n][i] * C[fuck][n - 1] % MOD) % MOD;
        } else all = (all + MOD + C[n][i] * C[fuck][n - 1] % MOD) % MOD;
    }
    return all;
}
int n, full;
void work() {
    LL ans = 0;
    for (int i = 0; i <= full; ++i) { // 枚举ave
        ans += n * calc(n - 1, full, i * n - i) % MOD;
        ans %= MOD;
    }
    cout << ans << endl;
}

int main() {
#ifdef local
    in();
#else
#endif
    C[0][0] = 1;
    C[1][0] = 1, C[1][1] = 1;
    for (int i = 2; i <= 14000; ++i) {
        int en = min(60, i);
        C[i][0] = 1;
        for (int j = 1; j <= en; ++j) {
            C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % MOD;
        }
    }
    while (scanf("%d%d", &n, &full) > 0 && (n + full)) work();
    return 0;
}