C++ :

#include<bits/stdc++.h>
using namespace std;
int n,m,a,b,c,d;
int main()
{
	cin>>n>>m>>a>>b>>c>>d;
	if(b*m>=n) {cout<<n;return 0;}
	else{
		int nn=n-b*m;
		if(c>=nn){cout<<b*m+nn*m;return 0;}
		else {cout<<-1;return 0;}
	}
}
/**
5 10 0 1 0 0
2 10 2 0 1 0
51 7 7 7 7 7 
*/