Problem:
Score : 200 points

Problem Statement

Find the largest square number not exceeding N. Here, a square number is an integer that can be represented as the square of an integer.

Constraints

1 \leq N \leq 10^9

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the largest square number not exceeding N.

Sample Input 1

10

Sample Output 1

9

10 is not square, but 9 = 3 × 3 is. Thus, we print 9.

Sample Input 2

81

Sample Output 2

81

Sample Input 3

271828182

Sample Output 3

271821169

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp1
{
    public static class MyExtensions
    {
        public static string Slice(this string str, int start = 0, int stop = 0)
        {
            if (start > str.Length || stop > str.Length || start < 0 || stop < 0)
                return "";

            if (stop == 0) stop = str.Length;
            return str.Substring(start, stop - start);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            B();
        }

        static void B()
        {
            var N = long.Parse(Read());
            for (var n = N; n > 0; n--)
            {
                var a = (int)Math.Sqrt(n);
                if (n == a*a)
                {
                    Console.WriteLine(n);
                    return;
                }
            }
        }

        public static IEnumerable<int> Range(int start, int stop)
        {
            if (start < 0 || stop < 0)
                throw new Exception("0未満");

            if (start > stop || (start == 0 && stop == 0))
                return new List<int>();

            return Enumerable.Range(start, stop - start);
        }

        public static bool IsDigit(string str)
        {
            var i = 0;
            return int.TryParse(str, out i);
        }

        public static int DigitsSum(long num)
        {
            return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
        }

        public static int[] ToIntArray(string str)
        {
            return str.Select(x => x.ToString()).Select(int.Parse).ToArray();
        }

        public static string Read()
        {
            return Console.ReadLine();
        }

        public static IList<int> ReadIntInputs()
        {
            return Read().Split(' ').Select(int.Parse).ToList();
        }

        public static int ReadInt()
        {
            return int.Parse(Read());
        }

        public static IList<string> ReadInputs()
        {
            return Read().Split(' ').ToList();
        }
    }
}