#include <bits/stdc++.h>
using namespace std;
int MAX(long long int a, long long int b) {
  if (a > b) {
    return a;
  } else {
    return b;
  }
}
int MIN(long long int a, long long int b) {
  if (a < b) {
    return a;
  } else {
    return b;
  }
}
int gcd(long long int a, long long int b) {
  long long int c;
  while (a != 0) {
    c = a;
    a = b % a;
    b = c;
  }
  return b;
}
int main() {
  int c, d, n, m, k;
  cin >> c;
  cin >> d;
  cin >> n;
  cin >> m;
  cin >> k;
  int ans = n * m;
  ans = ans - k;
  if (k <= 0) {
    cout << "0" << endl;
    return 0;
  }
  int a[105][105];
  int i, j;
  int min = 99999999;
  for (i = 0; i <= n * m; i++) {
    for (j = 0; j <= n * m; j++) {
      if (i * n + j >= ans) {
        if (c * i + d * j < min) {
          min = c * i + d * j;
        }
      }
    }
  }
  cout << min << endl;
  return 0;
}
