bzoj1224

题解：

暴力+剪纸

判断一下最大行不行，最小行不行

代码：

#include<bits/stdc++.h>
const double eps=1e-10;
using namespace std;
double s[55],q;
int n,m,x,y,ans;
void dfs(int x,int y,double p)
{
    if (p+s[x+(m-y)-1]-s[x-1]-q<-eps)return;
    if (p+s[n]-s[n-(m-y)]-q>eps)return;
    if (y==m){ans++;return;}
    dfs(x+1,y,p);
    dfs(x+1,y+1,p+1.0/x);
}
int main()
{
    scanf("%d%d%d%d",&m,&n,&x,&y);
    q=(double)x/y;
    for (int i=1;i<=n;i++) s[i]=s[i-1]+1.0/i;
    dfs(1,0,0.0);
    printf("%d",ans);
    return 0;
}