#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:60777216")
using namespace std;
int main() {
  int c, d;
  int n, m, k;
  cin >> c >> d;
  cin >> n >> m >> k;
  int rest = n * m - k;
  if (rest <= 0) {
    puts("0");
  } else {
    int ans = 1000000000;
    for (int i = 0; i <= 10000; ++i) {
      int r = rest - n * i;
      int l = (r <= 0 ? 0 : r);
      ans = min(ans, i * c + l * d);
    }
    printf("%d\n", ans);
  }
  return 0;
}
