#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[1000],m,N,sum=0;
    cin>>a[0]>>a[1]>>m>>N;
    for(int i=0;i<N;i++){
    	if(a[i]>=m){
    		break;
    	}
    	sum=sum+a[i];
    	a[i+2]=a[i]+a[i+1];
    }
    cout<<sum;
    return 0;
}