#include <bits/stdc++.h>
using namespace std;
const int N = 2009;
int main() {
  int n, m, c, d, k, cnt, i;
  while (~scanf("%d%d%d%d%d", &c, &d, &n, &m, &k)) {
    int tot = n * m - k;
    int ans = 0x3f3f3f3f;
    for (i = 0; n * i <= tot; ++i) {
      ans = min(ans, i * c + (tot - n * i) * d);
    }
    ans = min(ans, i * c);
    printf("%d\n", ans);
  }
  return 0;
}
