static bool Sirogin(int[] ar)
    {
        int[] ar2 = new int[ar.Length];
        bool valid = false;
        for (int i = 0; i < ar.Length - 2; i++)
            ar2[i] = ar[i] + ar[i + 2];
        for (int i = 0; i < ar.Length - 2; i++)
            if (ar2[i] == ar2[i + 2])
                valid = true;
            else
                valid = false;
        if (valid)
            return true;
        else
            return false;
    }
    static void Main(string[] args)
        {
            int[] a = new int[20];
            for (int i = 0; i < a.Length; i++)
                a[i] = int.Parse(Console.ReadLine());
            if (Sirogin(a))
                Console.WriteLine("array is sirogin");
            else
                Console.WriteLine("array is NOT sirogin");
            }