/* package whatever; // don't place package name! */
 
import java.util.*;
class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] arr = new int[n];
		for(int i=0; i<n; i++){
			arr[i] = sc.nextInt();
		}
		int d = arr[1] - arr[0];
		int a = arr[0];
		for(int i=0; i<n; i++){
			if(arr[i] == a+i*d){
				continue;
			}else{
				int ans = a+i*d;
				System.out.println(ans);
				return;
			}
		}
		System.out.println(a+n*d);
	}
}

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NgoxNCAxMiAxMCA4IDYgNA==

6
14 12 10 8 6 4