#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll a,n,m,x,k;
struct ff{
	ll sa;
	ll sk;
};
ff up[25],down[25];
ll p[25];
int main(){
    cin>>a>>n>>m>>x;
    up[1].sa=1;
    up[2].sk=1;
    down[2].sk=1;
    for(int i=3;i<n;i++){
		up[i].sa=up[i-1].sa+up[i-2].sa;
		down[i].sa=up[i-1].sa;
		up[i].sk=up[i-1].sk+up[i-2].sk;
		down[i].sk=up[i-1].sk;
	}
	int f1=up[n-1].sa*a-down[n-1].sa*a;
	int f2=up[n-1].sk-down[n-1].sk;
	k=(m-f1)/f2;
	for(int i=1;i<=n;i++){
		p[i]+=(up[i].sa-down[i].sa)*a;
		p[i]+=(up[i].sk-down[i].sk)*k;
	}
	cout<<p[x];
}

史山代码rt，1Re,3Wa,1Ac,感觉思路没问题