666

#include<bits/stdc++.h>
using namespace std;
double a1;
double a2;
double a3;
double a4;
double s;
int main(){
	cin>>a1>>a2>>a3>>a4;
	while(a2/(s+a1)>a3/a4){
		s++;
	}
	cout<<s; 
	return 0;
}