#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k, butuh, x, y, temp, ans;
int main() {
  scanf("%d%d", &c, &d);
  scanf("%d%d", &n, &m);
  scanf("%d", &k);
  butuh = n * m - k;
  ans = 1231231231;
  if (butuh > 0) {
    for (x = 0; x <= m; x++) {
      y = max(0, butuh - (n * x));
      temp = y * d + c * x;
      if (temp < ans) ans = temp;
    }
    printf("%d\n", ans);
  } else {
    printf("0\n");
  }
  return 0;
}
