#include<bits/stdc++.h>
using namespace std;
int d[20],f[20]={0};
int main(){
	int a,n,m,x;
	cin>>a>>n>>m>>x;
	int bus=0;
	while(f[n-1]!=m)
	{
		if(n<=2)
		{
			cout<<a<<endl;
			break;
		}
		d[1]=a;
		d[2]=bus;
		f[2]=a;
		for(int i=3;i<n;++i)
		{
			d[i]=d[i-1]+d[i-2];
			f[i]=d[i]+f[i-1]-d[i-1];
		}
		bus++;
	}
	cout<<f[x]<<endl;
	return 0;
}