#include <bits/stdc++.h>
using namespace std;
int main(){
  int A, B, C, D, E;
  cin >> A >> B >> C >> D >> E;
  int ans = A - B;
  for (int i = 1; i <= C; i++){
    ans = max(ans, A - B + (D - E) * i);
  }
  cout << ans << endl;
}