#include<bits/stdc++.h>
using namespace std;
int main(){
	int up[22],down[22],a,m,n,x,S=0;
	cin>>a>>n>>m>>x;
	up[1]=a;
	up[2]=down[2]=down[1]=up[n-1]=0;
	for(int i=3;i<=x;i++){
		up[i]=up[i-2]+up[i-1];
		down[i]=up[i-1];
	}
	for(int i=1;i<=x;i++){
		S+=up[i];
		S-=down[i];
	}
	cout<<S;
	return 0;
}

对了Sub3