#include <iostream>
#include <vector>
using namespace std;
int n;
vector<int> v;
int R0, W0, C, R;
int main() {
    while(cin >> R0 >> W0 >> C >> R, R0&&R0&&C&&R){
        int cnt=0;
        while(true){
            if(R0/W0>=C) break;
            cnt++;
            R0 += R;
        }
        cout << cnt << endl;
    }
    return 0;
}
