/*#include<iostream>
using namespace std;
int main()
{
    int n,a,m,x;
    cin>>a>>n>>m>>x;
    int sum1[30],sum2[30];
    sum1[2]=1,sum1[3]=2;
    if(x<=2)
    {
        cout<<a;
        return 0;
    }
    for(int i=4;i<n;i++)
    {
        sum1[i]=sum1[i-1]+sum1[i-2]-1;
        sum2[i]=sum2[i-1]+sum2[i-2]+1;
    }
    int b=(m-sum1[n-1]*a)/sum2[n-1];
    cout<<a*sum1[x]+b*sum2[x];
    return 0;
}*/
#include<cstdio>
using namespace std;
int sum1[100],sum2[100];
int main(){
	int a,n,m,x;
	scanf("%d%d%d%d",&a,&n,&m,&x);
	sum1[1]=1,sum1[2]=1,sum1[3]=2;
	for(int i=4;i<n;i++){
		sum1[i]=sum1[i-1]+sum1[i-2]-1;
		sum2[i]=sum2[i-1]+sum2[i-2]+1;
	}
	int b=(m-a*sum1[n-1])/sum2[n-1];
	printf("%d",a*sum1[x]+b*sum2[x]);
	return 0;
}