#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c;
  cin >> d;
  cin >> n;
  cin >> m;
  cin >> k;
  int remain = n * m - k;
  if (remain <= 0) {
    cout << 0;
    return 0;
  }
  int min = INT_MAX;
  for (int i = 0; i * n <= remain + n; i++) {
    int j = remain - i * n;
    if (j < 0) {
      j = 0;
    }
    int cur = i * c + j * d;
    if (cur < min) {
      min = cur;
    }
  }
  cout << min;
  return 0;
}
