rt

#include<bits/stdc++.h>
#define M 1000000000
#define ll long long
using namespace std;
int fibo(int x)
{
	long long f[M]={0};
	f[1]=1;
	f[2]=1;
	for(int i=3;i<=x;i++)
	{
		f[i]=f[i-1]+f[i-2];
	}
	return f[x];
}
long long fb[M];
int main()
{
	int n,a,m,x;
	cin>>a>>n>>m>>x;
	int last=m-a;
	int lasted=fibo(n-1)-fibo(n-2);
	int multiple=last*1.0/lasted;
	cout<<a+fibo(x-1)*multiple+fibo(x-2)*multiple;
	return 0;
}