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public class Checker
    {
        public bool CheckInn(long inn)
        {
            var values = inn.ToArray();
            switch (values.Length)
            {
                case 10:
                    #region Юр. лица
                    var coefficientsN10 = new byte[] { 2, 4, 10, 3, 5, 9, 4, 6, 8 };
                    int sumN10 = GetSumNx(values, coefficientsN10);
                    var checkNumberN10 = (sumN10 % 11) % 10;
                    return values[^1] == checkNumberN10;
                #endregion
                case 12:
                    #region Физ. лица
                    var coefficientsN11 = new byte[] { 7, 2, 4, 10, 3, 5, 9, 4, 6, 8 };
                    var coefficientsN12 = new byte[] { 3, 7, 2, 4, 10, 3, 5, 9, 4, 6, 8 };
                    var sumN11 = GetSumNx(values, coefficientsN11);
                    var checkNumberN11 = (sumN11 % 11) % 10;
                    var sumN12 = GetSumNx(values, coefficientsN12);
                    var checkNumberN12 = (sumN12 % 11) % 10;
                    return values[^2] == checkNumberN11 && values[^1] == checkNumberN12;
                #endregion
                default:
                    return false;
            }
        }
        private int GetSumNx(byte[] values, byte[] coefficientsNx)
        {
            var sumNx = 0;
            for (int i = 0; i < coefficientsNx.Length; i++)
                sumNx += coefficientsNx[i] * values[i];
            return sumNx;
        }
    }
 public static class Extensions
    {
        public static byte[] ToArray(this long number)
        {
            var values = new Stack<byte>(12);
            while (number != 0)
            {
                values.Push((byte)(number % 10));
                number /= 10;
            }
            return values.ToArray();
        }
    }