#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a,b,c;
	int s=0;
	cin>>a>>b;
	s=a+b;
	int m,n;
	cin>>m>>n;
	for(int i=3;i<=n;i++)
	{
		c=a+b;
		s=s+c;
		if(c>=m)
		{
			break;
		}
		a=b;
		b=c;
	}
	cout<<s;
	return 0;
}