#include <bits/stdc++.h>
int main() {
  int c, d, n, m, k;
  while (scanf("%d%d%d%d%d", &c, &d, &n, &m, &k) != EOF) {
    int sum = n * m - k;
    if (sum <= 0) {
      printf("0\n");
      continue;
    }
    int a = sum / n * c;
    if (sum % n) {
      int mod = sum % n;
      int a1 = a + c;
      int a2 = a + mod * d;
      a = a1 < a2 ? a1 : a2;
    }
    int b = sum * d;
    printf("%d\n", a = a < b ? a : b);
  }
  return 0;
}
