#include<iostream>
using namespace std;

int main(){
  int r0, w0, c, r;
  while( cin >> r0 >> w0 >> c >> r ) {
    if(r0 == 0 and w0 == 0 and  c == 0 and r == 0) break;

    int ans = 0;
    for(ans = 0; ; ans++){
      if( r0 + r*ans >= w0*c ) break;
    }
    cout << ans << endl;
  }
}