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

#define int long long

signed main() {
  int R0, W0, C, R;
  while ( cin >> R0 >> W0 >> C >> R, R0 ) {
    if ( W0*C <= R0 ) cout << 0 << endl;
    else cout << (W0*C-R0-1)/R+1 << endl;
  }
	 
  return 0;
}

