1 #include<iostream>
2 #include<stdio.h>
3 #include<cstring>
4 #include<cstdlib>
5 using namespace std;
6 typedef __int64 LL;
7 const LL mod = 1000000007;
8 LL cnm[5002][5002];
9 LL dp[5002][5002];
10
11 void prepare()
12 {
13 LL i,j;
14 for(i=0;i<=5000;i++)
15 {
16 cnm[0][i]=1;
17 cnm[i][0]=1;
18 }
19 for(i=1;i<=5000;i++)
20 {
21 for(j=1;j<=i&&j<=5000;j++)
22 {
23 if(i==j) { cnm[i][j]=1;continue;}
24 if(j==1) { cnm[i][j]=i;continue;}
25 cnm[i][j]=(cnm[i-1][j]+cnm[i-1][j-1])%mod;
26 }
27 }
28 dp[0][0]=1;
29 for(i=1;i<=5000;i++)
30 for(j=1;j<=5000;j++)
31 dp[i][j]=(dp[i-1][j-1]+((i-1)*dp[i-1][j])%mod)%mod;
32 }
33 int main()
34 {
35 LL n,a,b,cur;
36 LL l,r;
37 prepare();
38 while(scanf("%I64d%I64d%I64d",&n,&a,&b)>0)
39 {
40 if(n==0&&a==0&&b==0)break;
41 l=a;r=n-b+1;
42 for(cur=0;l<=r;l++)
43 {
44 cur=( cur+((cnm[n-1][l-1]*dp[l-1][a-1])%mod)*dp[n-l][b-1])%mod;
45 }
46 printf("%I64d\n",cur);
47 }
48 return 0;
49 }
