HEU 4048 Picking Balls

 1/**//**************************************
 2Problem: HEU 4048 Picking Balls
 3Time: 0.0000 s
 4Memory: 284 k 
 5Accepted Time: 2009-03-28 12:01:19
 6Tips: 求概率 
 7**************************************/
 8#include <stdio.h>
 9long long fun(int n,int m)
{
    int i,j;
    long long sum;
    if (m>n/2)m=n-m;
    sum=1;
    for(i=n-m+1,j=1;i<=n,j<=m;i++,j++)
        sum=sum*i/j;
    return sum;
}
int main()
{
    int n,k;
    while(1)
    {
        scanf("%d%d",&n,&k);
        if(n==-1&&k==-1)break;
        int i,a[100],b[100],sum=0;
        for(i=0;i<n;i++)
        {
            scanf("%d",&a[i]);
            sum+=a[i];
        }
        for(i=0;i<n;i++)scanf("%d",&b[i]);
        if(n==0)
        {
            if(k==0)printf("1.00000\n");
            else printf("0.00000\n");
            continue;
        }
        double sum1=(double)1.0/fun(sum,k);
        for(i=0;i<n;i++)
        {
            if(a[i]<b[i])break;    
            sum1*=fun(a[i],b[i]);
        }
        if(i==n)printf("%.5lf\n",sum1);
        else printf("0.00000\n");
    }
    return 0;
}