#include <bits/stdc++.h>
int solve(int c, int d, int n, int m, int k) {
  int s = n * m;
  if (k >= n * m) return 0;
  s = n * m - k;
  int sum = 0;
  while (1) {
    if (s <= n) {
      if (c < s * d)
        sum += c;
      else
        sum += s * d;
      break;
    }
    if (c < n * d)
      sum += c;
    else
      sum += n * d;
    s -= n;
  }
  return sum;
}
int main() {
  int c, d, n, m, k;
  while (scanf("%d%d%d%d%d", &c, &d, &n, &m, &k) != EOF) {
    int s = solve(c, d, n, m, k);
    printf("%d\n", s);
  }
  return 0;
}
