#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, w, x, c;
  cin >> a >> b >> w >> x >> c;
  cout << (c <= a ? 0 : c - a + (1ll * (c - a) * x - b + w - x - 1) / (w - x))
       << '\n';
  return 0;
}
