【题解】JSOI2011分特产

没sa可suo的，sui题一道……

#include <bits/stdc++.h>
using namespace std;
#define maxn 3000
#define mod 1000000007
#define int long long
int n, m, ans, a[maxn], C[maxn][maxn];

int read()
{
    int x = , k = ;
    char c; c = getchar();
    while(c < '' || c > '') { if(c == '-') k = -; c = getchar(); }
    while(c >= '' && c <= '') x = x *  + c - '', c = getchar();
    return x * k;
}

void Pre()
{
    for(int i = ; i < maxn; i ++) C[i][] = ;
    for(int i = ; i < maxn; i ++)
        for(int j = ; j < maxn; j ++)
            C[i][j] = (C[i - ][j - ] + C[i - ][j]) % mod;
}

int Up(int &x, int y) { x = (x + y) % mod; }
int Get(int x)
{
    int ret = ;
    for(int i = ; i <= m; i ++)
        ret = ret * C[n - x + a[i] - ][n - x - ] % mod;
    return ret;
}

signed main()
{
    n = read(), m = read(); Pre();
    for(int i = ; i <= m; i ++) a[i] = read();
    for(int i = ; i <= n; i ++)
    {
        int t = C[n][i] * Get(i) % mod;
        Up(ans, (i & ) ? -t : t);
    }
    printf("%lld\n", (ans + mod) % mod);
    return ;
}