#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
void Say(bool say, string a = "Yes", string b = "No"){cout << (say ? a : b) << endl;};

ll calc(ll R0, ll W0, ll C, ll R){
    for(ll x = 0; ; x++){
        if(R0 + x * R >= W0 * C)return x;
    }
}

int main(){
    for(ll R0, W0, C, R; cin >> R0 >> W0 >> C >> R, R0;){
        cout << calc(R0, W0, C, R) << endl;
    }
    return 0;
}
