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