#include <bits/stdc++.h>
using namespace std;
int n, m, k, c, d;
int dp[10000 + 5];
int flag[10000 + 5];
int rec(int x) {
  if (x <= 0) return 0;
  if (flag[x]) return dp[x];
  dp[x] = min(c + rec(x - n), d + rec(x - 1));
  flag[x] = 1;
  return dp[x];
}
int main() {
  cin >> c >> d >> n >> m >> k;
  cout << rec(n * m - k);
}
