#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  int ans = 2000000;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  int goal = n * m - k, t1 = -n, tans, tt;
  for (int i = 0; i <= m; i++) {
    t1 += n;
    tans = i * c;
    if (t1 >= goal) {
      ans = min(ans, tans);
      break;
    }
    ans = min(ans, tans + (goal - t1) * d);
  }
  printf("%d\n", ans);
  return 0;
}
