#include <bits/stdc++.h>
using namespace std;
int c, d;
int n, m;
int k, p;
int ans;
void solve() {
  int without_mod;
  if (d * n <= c) {
    without_mod = d * n;
  } else {
    without_mod = c;
  }
  ans = without_mod * (p / n);
  p %= n;
  if (d * p <= c) {
    ans += d * p;
  } else {
    ans += c;
  }
}
int main(int argc, char *argv[]) {
  scanf("%d%d", &c, &d);
  scanf("%d%d", &n, &m);
  scanf("%d", &k);
  p = n * m - k;
  p = max(p, 0);
  solve();
  printf("%d", ans);
  return 0;
}
