#include <bits/stdc++.h>
using namespace std;
long long a, b, w, x, c;
int main() {
  cin >> a >> b >> w >> x >> c;
  if (c <= a) {
    cout << 0;
    return 0;
  }
  long long ans = (long long)ceil(1.0 * (x * (c - a) - b) / (w - x));
  cout << ans + c - a << endl;
  return 0;
}
