var input = require('fs').readFileSync('/dev/stdin', 'utf8');
var arr = input.trim().split("\n");
while (true) {
   var [r0, w0, c, r] = arr.shift().split(" ").map(Number);
   if (r0 + w0 + c + r == 0) break;
   var cnt = 0;
   while (true) {
      if (r0 / w0 >= c) {
         console.log(cnt);
         break;
      }
      cnt++;
      r0 += r;
   }
}
