#include<iostream>
using namespace std;
int start_sum,station,final_sum,x;
int up,rnd=2;
int fib_a[25]={0,1,2,2,3,4,6,9,14,22,35,56,90,145,234,378,611,988,1598,2585,4182,6766,10947,17712};
int fib_u[25]={0,0,0,1,2,4,7,12,20,33,54,88,143,232,376,609,986,1596,2583,4180,6764,10945,17710,28656,46367};
int main()
{
    // freopen("fib.txt","w",stdout);
    scanf("%d %d %d %d",&start_sum,&station,&final_sum,&x);
    up=(final_sum-start_sum*fib_a[station])/fib_u[station];
    printf("%d",start_sum*fib_a[x]+up*fib_u[x]);
    return 0;
}