学步园

简单数学题，推出结果后直接计算即可

不取double 会wrong

#include<cstdio>
#include<cstring>
__int64 C(__int64 n, __int64 m)
{
 __int64 s = 1;
int i, j;
for (i=1,j=n; i<=m; i++, j--) 
  s = s * j / i;
return s;
}
int main()
{
int n,s;
int i,j,k;
while(scanf("%d%d",&n,&s)!=EOF)
    {
double sum=0,tsum;
if(s>n){ printf("100.00000\n");continue;}
if(s==0) {printf("0.00000\n");continue;}
for(i=0;i<=s;i++)
            {
                sum+=(double)C(n,i)*((__int64)1<<(n-i));
if(i==s-1) tsum=sum;
            }
            printf("%.5lf\n",100.0*tsum/sum);
    }
}