#include<bits/stdc++.h>
using namespace std;

int a[400];

int main(){
	int m,n,t=0,i=2;
	cin>>a[1]>>a[2]>>m>>n;
	t+=a[1];
	while(i<=n && a[i]<=m){
		t+=a[i];
		i++;
		a[i]=a[i-1]+a[i-2];
	}
	cout<<t;
	return 0;
}