baile.cpp

//
//  baile_de_formatura.cpp
//
//  Created by Lucca Siaudzionis on 19/11/14.
//  Copyright (c) 2014 Luccasiau. All rights reserved.
//
 
#include <cstdio>
#define MOD 1000000007
 
typedef long long lli;
 
#define MAXN 1010
 
int g, b;
lli fact[MAXN];
lli ways[MAXN][MAXN];
 
int main(){
     
    fact[0] = 1LL;
    for(int i = 1;i < MAXN;i++)
        fact[i] = (lli(i)*fact[i-1]) % MOD;
     
    for(int i = 1;i < MAXN;i++)
        ways[1][i] = 1LL,
        ways[i][i] = fact[i];
     
    for(int i = 2;i < MAXN;i++)
    for(int j = i+1;j < MAXN;j++)
        ways[i][j] = (lli(i)*( ways[i][j-1] + ways[i-1][j-1] )) % MOD;
     
    while(scanf("%d %d", &b, &g) && b) printf("%lld\n", ways[g][b]);
     
    return 0;
}