#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d;
  int n, m, k;
  scanf("%d %d %d %d %d", &c, &d, &n, &m, &k);
  int sum = n * m;
  if (sum <= k) {
    printf("0");
    return 0;
  } else {
    int t = 0;
    sum -= k;
    double a1 = c * 1.0 / n;
    double a2 = d * 1.0 / 1;
    if (a1 >= a2)
      t = sum * d;
    else {
      int turn = sum / n;
      t = c * turn;
      sum -= turn * n;
      if (d * sum >= 1 * c)
        t += c;
      else
        t += d * sum;
    }
    printf("%d", t);
  }
  return 0;
}
