#include<bits/stdc++.h>
using namespace std;
double n,k,a,b,s; 
int main(){
    cin>>n>>k>>a>>b;
    if(a == 1)
    {
        cout<< 0 ;
        return 0;
    }
    else if(b / a == n / k)
    {
        cout<< 0 ;
        return 0;
    }
    while(b / a > n / k)
    {
        s++;
        n++;
    }
    cout<< ceil(s) ;
	return 0;
}