#include<bits/stdc++.h>
using namespace std;
long long a,n,m,x,b[200],c[200],z[200],mid;
void pro(int x) {
	b[2]=x;
	c[2]=x;
	z[2]=a;
	for (int i=3; i<=n-1; i++) {
		b[i]=b[i-1]+b[i-2];
		c[i]=b[i-1];
		z[i]=z[i-1]+b[i]-c[i];
	}
}
int main() {
	cin>>a>>n>>m>>x;
	long long l=0,r=100000000;
	b[1]=a;
	c[1]=0;
	z[1]=a;
	while (l<=r) {
		mid=l+(r-l)/2;
		pro(mid);
		if (z[n-1]<m) l=mid+1;
		else if (z[n-1]>m) r=mid-1;
		else break;
	}
	cout<<z[x];
}