传送门

题意：

思路：

显然是多重集排列数，我们考虑构造指数生成函数，让后模拟一下多项式乘法即可啦。
由于存在分数，所以直接用$double$即可。

//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4.1,sse4.2,avx,avx2,popcnt,tune=native")
//#pragma GCC optimize(2)
#include<cstdio>
#include<iostream>
#include<string>
#include<cstring>
#include<map>
#include<cmath>
#include<cctype>
#include<vector>
#include<set>
#include<queue>
#include<algorithm>
#include<sstream>
#include<ctime>
#include<cstdlib>
#include<random>
#include<cassert>
#define X first
#define Y second
#define L (u<<1)
#define R (u<<1|1)
#define pb push_back
#define mk make_pair
#define Mid ((tr[u].l+tr[u].r)>>1)
#define Len(u) (tr[u].r-tr[u].l+1)
#define random(a,b) ((a)+rand()%((b)-(a)+1))
#define db puts("---")
using namespace std;//void rd_cre() { freopen("d://dp//data.txt","w",stdout); srand(time(NULL)); }
//void rd_ac() { freopen("d://dp//data.txt","r",stdin); freopen("d://dp//AC.txt","w",stdout); }
//void rd_wa() { freopen("d://dp//data.txt","r",stdin); freopen("d://dp//WA.txt","w",stdout); }typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int,int> PII;const int N=1000010,mod=1e9+7,INF=0x3f3f3f3f;
const double eps=1e-6;int n,m;
int a[N];
double x[N],y[N];int fun(int x) {int ans=1;for(int i=1;i<=x;i++) ans*=i;return ans;
}int main()
{
//	ios::sync_with_stdio(false);
//	cin.tie(0);while(cin>>n>>m) {for(int i=1;i<=n;i++) cin>>a[i];for(int j=0;j<=m;j++) x[j]=y[j]=0;for(int i=0;i<=a[1];i++) x[i]=1.0/fun(i);for(int i=2;i<=n;i++) {for(int j=0;j<=m;j++) {for(int k=0;k+j<=m&&k<=a[i];k++) {(y[j+k]+=x[j]/fun(k));}}for(int j=0;j<=m;j++) x[j]=y[j],y[j]=0;}printf("%.0f\n",(x[m]*fun(m)));}return 0;
}
/**/