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