样例过了……

#include<bits/stdc++.h>
using namespace std;
int n,k,p,q;
double a;
int ans;
int main(){
	scanf("%d%d%d%d",&n,&k,&p,&q);
	a = 1.0*p/q/1.0;
	ans = ceil(k/a)-n;
	printf("%d",ans);
	return 0;
}