#!/usr/local/bin/tcc -run
#include<stdio.h>
int main(void)
{
	int i,x,a,b,c,d;
	int nae[100];
	
	scanf("%d",&x);
	for(i=0;i<x+1;i++) scanf("%d",&nae[i]);
	for(i=0;i<x+1;i++){
		a=nae[i+1]-nae[i];
		b=nae[i+2]-nae[i+1];
		c=nae[i+3]-nae[i+2];
		d=nae[i+2]-nae[i];
		if (a!=b&&b==c&&a!=d){printf("%d\n",nae[i]); break;}
		if (a!=b&&b!=c&&c==d){printf("%d\n",nae[i+1]); break;}
		if (a!=b&&b!=c&&a!=d){printf("%d\n",nae[i+2]); break;}
		if (a==b&&a!=c&&b!=c){printf("%d\n",nae[i+3]); break;}
	}
	return 0;
}