#include<iostream>
using namespace std;
int a,b,m,n,c[370];
int main(){
    cin >> a >> b >> m >> n;
    int sum = a + b;
    c[1] = a;
    c[2] = b;
    for(int i = 3;i <= n;i++){
        c[i] = c[i - 1] + c[i - 2];
        sum += c[i];
        if(c[i] >= m){
            break;
        }
    }
    cout << sum;
    return 0;
}