#include <bits/stdc++.h>
using namespace std;
int dp[100001];
int c, d, n, m, k;
int solve(int x) {
  if (x <= 0) return 0;
  if (dp[x] != 0) return dp[x];
  dp[x] = min(solve(x - n) + c, solve(x - 1) + d);
  return dp[x];
}
int main() {
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  int ans = 0;
  if (k >= n * m)
    ans = 0;
  else {
    int a = n * m - k;
    ans += solve(a);
  }
  cout << ans;
}
