#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  while (~scanf("%d%d%d%d%d", &c, &d, &n, &m, &k)) {
    int left = n * m - k;
    int res = 0;
    if (left >= 0) {
      if (n * d > c) {
        while (left > n) {
          res += c;
          left -= n;
        }
        if (left * d <= c)
          res += d * left;
        else
          res += c;
      } else {
        res += left * d;
      }
    }
    printf("%d\n", res);
  }
  return 0;
}
