#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k;
int ans = 2147483647, dp[10000001] = {0};
int main(void) {
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  if (k >= n * m)
    printf("0\n");
  else {
    for (int i = 1; i <= n * m; i++) dp[i] = min(dp[i - n] + c, dp[i - 1] + d);
    for (int i = n * m - k; i <= n * m; i++) ans = min(ans, dp[i]);
    printf("%d\n", ans);
  }
  return 0;
}
