#include <bits/stdc++.h>
using namespace std;
int f[100100] = {1 << 30};
int n, m, c, d, k;
int main() {
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  f[0] = 0;
  for (int i = 1; i <= n * m; i++)
    if (n <= i)
      f[i] = min(f[i - n] + c, f[i - 1] + d);
    else
      f[i] = f[i - 1] + d;
  int ans = 0x3f3f3f3f;
  for (int i = n * m - k; i <= n * m; i++) ans = min(ans, f[i]);
  printf("%d", ans);
}
