Problem:
Score : 200 points

Problem Statement

We have a string S of length N consisting of uppercase English letters.

How many times does ABC occur in S as contiguous subsequences (see Sample Inputs and Outputs)?

Constraints

3 \leq N \leq 50

S consists of uppercase English letters.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print number of occurrences of ABC in S as contiguous subsequences.

Sample Input 1

10
ZABCDBABCQ

Sample Output 1

2

Two contiguous subsequences of S are equal to ABC: the 2-nd through 4-th characters, and the 7-th through 9-th characters.

Sample Input 2

19
THREEONEFOURONEFIVE

Sample Output 2

0

No contiguous subsequences of S are equal to ABC.

Sample Input 3

33
ABCCABCBABCCABACBCBBABCBCBCBCABCB

Sample Output 3

5

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

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

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = Io.Read<int>();
            var S = Io.Read();

            var ss = "";
            var numABC = 0;
            foreach (var c in S)
            {
                if (ss == "" && c == 'A')
                {
                    ss += c;
                }
                else if (ss == "A" && c == 'B')
                {
                    ss += c;
                }
                else if (ss == "AB" && c == 'C')
                {
                    ss = "";
                    numABC++;
                }
                else
                {
                    ss = "";
                }
            }

            Io.Write(numABC.ToString());
        }

        static int NumberOfDigits(long x)
        {
            if (x < 10) return 1;

            var result = 0;
            while (x > 0)
            {
                result++;
                x /= 10;
            }

            return result;
        }

        static int SumOfDigits(int x)
        {
            var sum = 0;
            while (x > 0)
            {
                sum += x % 10;
                x /= 10;
            }

            return sum;
        }

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

            public static IEnumerable<string> ReadRepeat(int count)
            {
                return Enumerable.Range(0, count).Select(_ => Read());
            }

            public static T Read<T>()
            {
                return (T)Convert.ChangeType(Read(), typeof(T));
            }

            public static IEnumerable<T> ReadRepeat<T>(int count)
            {
                return Enumerable.Range(0, count).Select(_ => Read<T>());
            }

            public static string[] ReadList(char devide = ' ')
            {
                return Read().Split(devide);
            }

            public static T[] ReadList<T>(char devide = ' ')
            {
                return ReadList(devide)
                    .Select(s => (T)Convert.ChangeType(s, typeof(T)))
                    .ToArray();
            }

            public static IEnumerable<T[]> ReadListRepeat<T>(int count, char devide = ' ')
            {
                return Enumerable.Range(0, count).Select(_ => ReadList<T>(devide));
            }

            public static void Write(string value)
            {
                Console.WriteLine(value);
            }
        }
    }
}