#include<bits/stdc++.h>
using namespace std;
int n,m,a,b,ans;  
int main() {
	cin>>a>>b>>m>>n;
	ans=a+b;
	while(n--){
		if(a+b>=m) break;
		ans+=a+b;
		b=a+b;
		a=b-a;
	}
	if(a+b>=m){
		ans+=a+b;
	}
	cout<<ans;
    return 0;
}