#include <bits/stdc++.h>
using namespace std;
int main() {
    int R0,W0,C,R;
    while(scanf("%d %d %d %d",&R0,&W0,&C,&R),R0) {
        int n=0;
        if(W0*C>R0) {
            n=(int)ceil((W0*C-R0)/(double)R);
        }
        printf("%d\n",n);
    }
}