#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e4 + 5;
int f[maxn], a, b, n, m, k, C;
void bbINF(int w, int v) {
  for (int i = w; i <= C; ++i) f[i] = min(f[i], f[i - w] + v);
}
int main() {
  scanf("%d%d%d%d%d", &a, &b, &n, &m, &k);
  memset(f, 0x7f, sizeof(f));
  f[0] = 0;
  C = n * m;
  bbINF(n, a);
  bbINF(1, b);
  printf("%d\n", *min_element(f + C - k, f + C + 1));
  return 0;
}
