Problem:
Score : 100 points

Problem Statement

There are N students in a school.

We will divide these students into some groups, and in each group they will discuss some themes.

You think that groups consisting of two or less students cannot have an effective discussion, so you want to have as many groups consisting of three or more students as possible.

Divide the students so that the number of groups consisting of three or more students is maximized.

Constraints

1 \leq N \leq 1000

All input values are integers.

Input

Input is given from Standard Input in the following format:

N

Output

If you can form at most x groups consisting of three or more students, print x.

Sample Input 1

8

Sample Output 1

2

For example, you can form a group of three students and another of five students.

Sample Input 2

2

Sample Output 2

0

Sometimes you cannot form any group consisting of three or more students, regardless of how you divide the students.

Sample Input 3

9

Sample Output 3

3

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

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

namespace Contest
{
  class Program
  {
    static void Main(string[] args)
    {
      ABC089.A();
    }
    class ABC088
    {
      public static void A()
      {
        var N = Read.Int();
        var A = Read.Int();
        Console.WriteLine(N % 500 <= A ? "Yes" : "No");
      }
      public static void B()
      {
        var N = Read.Int();
        var A = Read.IntN().OrderBy(x => -x).ToArray();
        Console.WriteLine(A.Select((x, i) => i % 2 == 0 ? x : -x).Sum());
      }
      public static void C()
      {
        var C = Read.LongMatrix(3, 3);
        bool ok = true;
        for(int i = 0; i < 2; ++i)
        {
          if (C[i, 0] - C[i + 1, 0] != C[i, 1] - C[i + 1, 1]) { ok = false; }
          if (C[i, 0] - C[i + 1, 0] != C[i, 2] - C[i + 1, 2]) { ok = false; }
          if (C[i, 1] - C[i + 1, 1] != C[i, 2] - C[i + 1, 2]) { ok = false; }
          if (C[0, i] - C[0, i + 1] != C[1, i] - C[1, i + 1]) { ok = false; }
          if (C[0, i] - C[0, i + 1] != C[2, i] - C[2, i + 1]) { ok = false; }
          if (C[1, i] - C[1, i + 1] != C[2, i] - C[2, i + 1]) { ok = false; }
        }
        Console.WriteLine(ok ? "Yes" : "No");
      }
      public static void D()
      {
        var H = Read.Int();
        var W = Read.Int();
        var S = Enumerable.Range(0, H).Select(i => Read.Str()).ToArray();
        var board = new bool[H + 2, W + 2];
        int white = 0;
        for(int i = 0; i < H; ++i)
        {
          for(int j = 0; j < W; ++j)
          {
            board[i + 1, j + 1] = S[i][j] == '.';
            if(S[i][j] == '.') { white++; }
          }
        }
        var dx = new int[] { 1, -1, 0, 0 };
        var dy = new int[] { 0, 0, 1, -1 };
        var dist = Util.Gen(H + 2, W + 2, -1);
        dist[1, 1] = 0;
        var queue = new Queue<Tuple<int, int>>();
        queue.Enqueue(new Tuple<int, int>(1, 1));
        while (queue.Count > 0)
        {
          var t = queue.Dequeue();
          for(int i = 0; i < 4; ++i)
          {
            var x = t.Item1 + dx[i];
            var y = t.Item2 + dy[i];
            if (dist[x, y] < 0 && board[x, y])
            {
              dist[x, y] = dist[t.Item1, t.Item2] + 1;
              queue.Enqueue(new Tuple<int, int>(x, y));
            }
          }
        }
        if (dist[H, W] < 0) { Console.WriteLine(-1);return; }
        Console.WriteLine(white - 1 - dist[H, W]);
      }
    }
    class ABC089
    {
      public static void A()
      {
        var N = Read.Int();
        Console.WriteLine(N / 3);
      }
      public static void B()
      {
        var N = Read.Int();
        var S = Read.StrN();
        Console.WriteLine(S.Contains("Y") ? "Four" : "Three");
      }
      public static void C()
      {
        var N = Read.Int();
        var S = Enumerable.Range(0, N).Select(i => Read.Str()).ToArray();
        var count = new long[5];
        foreach(var s in S)
        {
          switch (s[0])
          {
            case 'M': count[0]++; break;
            case 'A': count[1]++; break;
            case 'R': count[2]++; break;
            case 'C': count[3]++; break;
            case 'H': count[4]++; break;
          }
        }
        long sum = 0;
        for(int i = 0; i < 5; ++i)
        {
          for(int j = i + 1; j < 5; ++j)
          {
            for(int k = j + 1; k < 5; ++k)
            {
              sum += count[i] * count[j] * count[k];
            }
          }
        }
        Console.WriteLine(sum);
      }
      public static void D()
      {
        var H = Read.Int();
        var W = Read.Int();
        var D = Read.Int();
        var A = Read.LongMatrix(H, W);
        var place = new Tuple<int, int>[H * W + 1];
        for(int i = 0; i < H; ++i)
        {
          for(int j = 0; j < W; ++j)
          {
            place[(int)A[i, j]] = new Tuple<int, int>(i, j);
          }
        }
        var Q = Read.Int();
        var LR = Read.TupleInt(Q);
        var distance = new List<int>[D];
        for(int i = 0; i < D; ++i)
        {
          var list = new List<int>();
          distance[i] = list;
          list.Add(0);
          for (int j = 1; ; ++j)
          {
            var prev = i + D * (j - 1);
            var next = i + D * j;
            if (next > H * W) { break; }
            if (prev == 0) { list.Add(0); continue; }
            var d = Math.Abs(place[prev].Item1 - place[next].Item1) + Math.Abs(place[prev].Item2 - place[next].Item2);
            list.Add(list.Last() + d);
          }
        }
        var ans = new List<int>();
        foreach(var lr in LR)
        {
          var c = lr.Item1 % D;
          var s = lr.Item1 / D;
          var t = lr.Item2 / D;
          ans.Add(distance[c][t] - distance[c][s]);
        }
        Console.WriteLine(String.Join(Environment.NewLine, ans));
      }
    }


    class Util
    {
      public static T[,] Gen<T>(int r, int c, T v)
      {
        var matrix = new T[r, c];
        for (int i = 0; i < r; ++i)
        {
          for (int j = 0; j < c; ++j)
          {
            matrix[i, j] = v;
          }
        }
        return matrix;
      }
      public static long[] ASum(long[] array)
      {
        var res = new long[array.Length + 1];
        for (int i = 0; i < array.Length; ++i)
        {
          res[i + 1] = res[i] + array[i];
        }
        return res;
      }
      public static long[,] ASum(long[,] matrix)
      {
        var N = matrix.GetLength(0);
        var M = matrix.GetLength(1);
        var res = new long[N + 1, M + 1];
        for(int i = 0; i < N; ++i)
        {
          for(int j = 0; j < M; ++j)
          {
            res[i + 1, j + 1] = res[i, j + 1] + res[i + 1, j] - res[i, j] + matrix[i, j];
          }
        }
        return res;
      }
      public static Dictionary<T, long> Frequency<T>(T[] array)
      {
        var dict = new Dictionary<T, long>();
        foreach (var a in array)
        {
          if (!dict.ContainsKey(a)) { dict.Add(a, 0); }
          dict[a]++;
        }
        return dict;
      }
      public static bool IsPrime(long x)
      {
        var fact = Factorize(x);
        return fact.Count == 1 && fact.Values.ElementAt(0) == 1;
      }
      public static Dictionary<long, int> Factorize(long x)
      {
        var dict = new Dictionary<long, int>();
        long minp = 2;
        while (x > 1)
        {
          bool ok = false;
          for (long p = minp; p * p <= x; ++p)
          {
            if (x % p == 0)
            {
              if (!dict.ContainsKey(p))
              {
                dict.Add(p, 0);
              }
              dict[p]++;
              minp = p;
              x /= p;
              ok = true;
              break;
            }
          }
          if (!ok)
          {
            if (!dict.ContainsKey(x)) { dict.Add(x, 0); }
            dict[x]++;
            break;
          }
        }
        return dict;
      }
    }

    static class Read
{
static Queue<string> queue = new Queue<string>();
static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
public static ulong[] ULongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToUInt64(s)).ToArray(); }
public static string[] StrN() { return Console.ReadLine().Split(' '); }
public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
public static long[,] LongMatrix(int r, int c) { var mat = new long[r, c]; for (int i = 0; i < r; ++i) { var x = Read.LongN(); for (int j = 0; j < c; ++j) { mat[i, j] = x[j]; } } return mat; }
public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
{
Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
        for (int i = 0; i < M; ++i) { var ab = Read.IntN(); if (!dict[ab[0]].Contains(ab[1])) { dict[ab[0]].Add(ab[1]); } if (!isDirectional && !dict[ab[1]].Contains(ab[0])) { dict[ab[1]].Add(ab[0]); } }
        return dict;
}
public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
{
Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>(); for (int i = 0; i <= N; ++i) { dict.Add(i, new Dictionary<int, long>()); }
for (int i = 0; i < M; ++i) { var ab = Read.IntN(); dict[ab[0]].Add(ab[1], ab[2]); dict[ab[1]].Add(ab[0], ab[2]); }
return dict;
}
}
    }
}