180105_10707_수도요금

//https://www.acmicpc.net/problem/10707 #include <iostream>

using namespace std;

int main()

{

int a,b,c,d,p;

cin >>a >> b >> c>> d>> p;

int costx=a*p;

int costy=(p>c ?b+d*(p-c) :b );

if(costx>costy) cout << costy;

else cout << costx;

}