Language: C

//
// ABC163A
// URL : https://atcoder.jp/contests/abc162/tasks/abc163_a
// 
using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;

namespace AtCoder
{
    class ABC163A
    {
        static void Main(string[] args)
        {
            preprocess();


            // 整数の入力
            long r = long.Parse(Console.ReadLine());

            var ans = r * 2 * Math.PI;

            Console.WriteLine(ans.ToString());
            Console.Out.Flush();
        }

        //---------------------------------------------------------------------------------
        /// <summary>
        /// 
        /// </summary>
        public static void preprocess()
        {
            //出力高速化
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
        }


        /// <summary>
        /// 与えられた数が素数かどうかを計算する
        /// </summary>
        /// <param name="num">対象数</param>
        /// <returns>true:素数, false:素数ではない(合成数)</returns>
        public static bool isPrime(int num)
        {
            if (num < 2) return false;
            else if (num == 2) return true;
            else if (num % 2 == 0) return false;

            double sqrtNum = Math.Sqrt(num);
            for (int i = 3; i <= sqrtNum; i += 2)
            {
                if (num % i == 0)
                {
                    return false;
                }
            }
            return true;
        }

        /// <summary>
        /// 順列mPnを計算する
        /// </summary>
        /// <param name="m"></param>
        /// <param name="n"></param>
        /// <returns></returns>
        public static long Permutation(long m, long n)
        {
            long ret = 1;
            for(var i = 0; i < n; ++i)
            {
                ret *= m - i;
            }
            return ret;
        }
        /// <summary>
        /// 組み合わせmCnを計算する
        /// </summary>
        /// <param name="m"></param>
        /// <param name="n"></param>
        /// <returns></returns>
        public static long Combination(long m, long n)
        {
            if(m == n || n == 0)
            {
                return 1;
            }
            long ans = 1;
            for(var i = 0; i <= n; ++i)
            {
                ans *= (m - i + 1) / i;
            }
            return ans;
        }

        static long Gcd(long m, long n)
        {
            if (n == 0) return m;
            return Gcd(n, m % n);
        }

        static long Lcm(long m, long n)
        {
            var gcd = Gcd(m, n);
            //return (m * n) / gcd;
            return m / gcd * n;//桁あふれ対策
        }
    }
}



Compiler output:
main.c:5:1: error: unknown type name 'using'
    5 | using System;
      | ^~~~~
main.c:6:1: error: unknown type name 'using'
    6 | using System.Collections.Generic;
      | ^~~~~
main.c:6:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    6 | using System.Collections.Generic;
      |             ^
main.c:7:1: error: unknown type name 'using'
    7 | using System.Linq;
      | ^~~~~
main.c:7:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    7 | using System.Linq;
      |             ^
main.c:8:1: error: unknown type name 'using'
    8 | using System.IO;
      | ^~~~~
main.c:8:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    8 | using System.IO;
      |             ^
main.c:9:1: error: unknown type name 'using'
    9 | using System.Text;
      | ^~~~~
main.c:9:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    9 | using System.Text;
      |             ^
main.c:11:1: error: unknown type name 'namespace'
   11 | namespace AtCoder
      | ^~~~~~~~~
main.c:12:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
   12 | {
      | ^