#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d;
  while (scanf("%d%d", &c, &d) != EOF) {
    int n, m, k;
    scanf("%d%d%d", &n, &m, &k);
    int sum = n * m;
    int left = sum - k;
    if (left <= 0)
      printf("0\n");
    else {
      int po1, po2;
      if (c * 1.0 / d < n) {
        po1 = floor(left * 1.0 / n);
        po2 = ceil(left * 1.0 / n);
        if (po1 * n + 1 < left) {
          int h = left - po1 * n;
          printf("%d", min(po2 * c, po1 * c + h * d));
        } else
          printf("%d\n", min(po2 * c, po1 * c + d));
      } else {
        printf("%d\n", left * d);
      }
    }
  }
  return 0;
}
