#include<bits/stdc++.h>
using namespace std;

int main(){
    int R0, W0, C, R;
    long ans;

    while(true){
        cin >> R0 >> W0 >> C >> R;
        ans = 0;
        if(!(R0 || W0 || C || R)) break;
        while(true){
            if(R0 + ans*R - C*W0 >= 0) break;
            ans++;
        }

        cout << ans << "\n";
    }
    return 0;
}
