Problem:
Score : 300 points

Problem Statement

There is a circular pond with a perimeter of K meters, and N houses around them.

The i-th house is built at a distance of A_i meters from the northmost point of the pond, measured clockwise around the pond.

When traveling between these houses, you can only go around the pond.

Find the minimum distance that needs to be traveled when you start at one of the houses and visit all the N houses.

Constraints

2 \leq K \leq 10^6

2 \leq N \leq 2 \times 10^5

0 \leq A_1 < ... < A_N < K

All values in input are integers.

Input

Input is given from Standard Input in the following format:

K N
A_1 A_2 ... A_N

Output

Print the minimum distance that needs to be traveled when you start at one of the houses and visit all the N houses.

Sample Input 1

20 3
5 10 15

Sample Output 1

10

If you start at the 1-st house and go to the 2-nd and 3-rd houses in this order, the total distance traveled will be 10.

Sample Input 2

20 3
0 5 15

Sample Output 2

10

If you start at the 2-nd house and go to the 1-st and 3-rd houses in this order, the total distance traveled will be 10.

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Windows;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Atcoder
{
    public class Program
    {
        static void Main(string[] args)
        {
            var r = ReadSplit();
            var K = r[0].ToInt();
            var N = r[1].ToInt();
            var r2 = ReadSplit();
            var array1 = new List<int>();
            var array2 = new List<int>();
            for (int i = 0; i < N; i++)
            {
                var half = (decimal)((decimal)K / (decimal)2);
                var a = r2[i].ToInt();
                if (a >= half)
                {
                    array1.Add(a - K);
                }
                else
                {
                    array1.Add(a);
                }
            }

            for (int i = 0; i < N; i++)
            {
                var a = r2[i].ToInt();
                array2.Add(a);
            }

            var ans1 = Math.Abs(array1.Max() - array1.Min());
            var ans2 = Math.Abs(array2.Max() - array2.Min());
            var ans = Math.Min(ans1, ans2);
            Write(ans);
        }

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

        public static string[] ReadSplit()
        {
            return Console.ReadLine().Split(' ');
        }

        public static void Write(object str)
        {
            Console.WriteLine(str);
            Read();
        }
    }
}


public static class Extentions
{
    public static int ToInt(this object obj)
    {
        return int.Parse(obj.ToString());
    }

    public static decimal ToDecimal(this object obj)
    {
        return decimal.Parse(obj.ToString());
    }

    public static double ToDouble(this object obj)
    {
        return double.Parse(obj.ToString());
    }
}