#include <bits/stdc++.h>
using namespace std;
int dp[10005], C;
void bbINF(int v, int w) {
  for (int i = w; i <= C; ++i) dp[i] = min(dp[i], dp[i - w] + v);
}
int main() {
  int c, d, n, m, k;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  C = n * m;
  for (int i = 1; i <= C; ++i) dp[i] = 1e8;
  bbINF(c, n);
  bbINF(d, 1);
  int ans = 1e8;
  for (int i = C - k; i <= C; ++i) ans = min(ans, dp[i]);
  printf("%d\n", ans);
  return 0;
}
