#include <stdio.h>

int main() {
	int n ; 
	printf("enter the value of n\n");
	scanf("%d", &n);
	int arr [n];
	for (int i = 0; i <n; i++)
	{
	  scanf("%d", &arr[i]);
	}
	int flag=0;
	for (int i = 1; i<n-1; i++)
	{
		if(2*arr[i]==arr[i-1]+arr[i+1])
		{
		  flag=1;
		}
		else
		{
			flag=0;
		}
	
	}
	if(flag==1)
	{
		printf("it is an A.P \n");
	}
	else { 
		printf( " it is not an A.P");		
	}
	
	
	
}