hnoi2002 彩票

#include <cstdlib>
#include <algorithm>
#include <cstring>
#include <cctype>
#define INF 2000000000
using namespace std;
typedef long long ll;
int n,m,x,y,ans=0;
double sum[55]={0},lim,EPS=1e-11;
int (){
    int f=1,x=0;char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')f=-f;c=getchar();}
    while(c>='0'&&c<='9')x=x*10+c-'0',c=getchar();
    return f*x; 
}
void dfs(int at,int lst,double t){
    
    if(at==n){
        if(t-lim<EPS&&t-lim>-EPS)ans++;
        return ;
    }
    if(t+sum[m]-sum[m-n+at]-EPS>lim)return ;
    //加了最小的还是超过了
    for(int i=lst+1;i<=m-n+at+1;i++){
        if(t+sum[i+n-at-1]-sum[i-1]+EPS<lim)return ;
        //加了最大的还是达不到 
        dfs(at+1,i,t+1.0/(double)i);
    }
    
}
void init(){
    n=read(),m=read(),x=read(),y=read();
    for(int i=1;i<=m;i++)
        sum[i]=sum[i-1]+1.0/(double)i;
    lim=x,lim/=(double)y;
}
void solve(){
    for(int i=1;i<=m-n+1;i++)
        dfs(1,i,1.0/(double)i);
    printf("%dn",ans);
}
int main(){
    init();
    solve();
    return 0;
}