样例都没过......

#include<bits/stdc++.h>
using namespace std;

int main(){
	std::ios::sync_with_stdio(0);
	int a,n,m,x,num=0,xx,yy;
	cin>>a>>n>>m>>x;
	int b[n+1],y,c[n+1];
	b[1]=a,b[2]=a,b[3]=2*a,b[4]=2*a+y,b[n]=m;
	c[1]=a,c[2]=y,c[3]=a,c[4]=y;
	for(int i=5;i<=n;i++){
		c[i]=c[i-1]+c[i-2];
		if(i!=n) b[i]=b[i-1]+c[i];
	}
	for(int i=1;;i++){
		for(int ii=1;ii<=n;ii++){
			if(i*a+ii*y==b[n-1]+c[n]){
				num++;
				xx=i;yy=ii;
				break;
			}
		}
		if(num==1) break;
	}
	y=(m-xx*a)/yy;
	cout<<b[x];
}