#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const int _inf = 0xc0c0c0c0;
const long long INF = 0x3f3f3f3f3f3f3f3f;
const long long _INF = 0xc0c0c0c0c0c0c0c0;
const long long mod = (int)1e9 + 7;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long ksm(long long a, long long b, long long mod) {
  int ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return ans;
}
long long inv2(long long a, long long mod) { return ksm(a, mod - 2, mod); }
void exgcd(long long a, long long b, long long &x, long long &y, long long &d) {
  if (!b) {
    d = a;
    x = 1;
    y = 0;
  } else {
    exgcd(b, a % b, y, x, d);
    y -= x * (a / b);
  }
}
int dp[10025];
int main() {
  int c, d, n, m, k, V, ans = inf;
  scanf("%d%d", &c, &d);
  scanf("%d%d", &n, &m);
  scanf("%d", &k);
  memset(dp, 0x3f, sizeof(dp));
  dp[0] = 0;
  for (int j = n; j <= n * m; ++j) {
    dp[j] = min(dp[j], dp[j - n] + c);
  }
  for (int j = 1; j <= n * m; ++j) {
    dp[j] = min(dp[j], dp[j - 1] + d);
  }
  for (int i = n * m - k; i <= n * m; ++i) {
    ans = min(ans, dp[i]);
  }
  printf("%d\n", ans);
  return 0;
}
