#include <bits/stdc++.h>
using namespace std;
int c,x,n,m,av,uv,as,us,t,a[23] = {0,1,0},u[23] = {0,0,1};
int main(){
	cin >> c >> n >> m >> x;
	av = uv = as = us = 1;
	if(x == 1 || x == 2){ cout << c; return 0;}
	for(int i = 3;i <= n;i ++){
		a[i] = u[i - 1];
		u[i] = a[i - 1] + u[i - 1];
		as += a[i]; us += u[i];
		if(i <= x){ av += a[i]; uv += u[i];}
	}
	cout << av * c + uv * (m - c - as * c) / us + c << endl;
	return 0;
}