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: Accepted)

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

namespace AtCoder
{
    static class Program
    {
        static void Main()
        {
            //ここから
            var N = IO.ReadLineToInt();
            var S = IO.ReadLineToString();
            var sum = 0;

            for(int i = 0; i < N - 2; i++)
            {
                if (S[i] != 'A') continue;
                if (S[i + 1] != 'B') continue;
                if (S[i + 2] != 'C') continue;
                sum++;
            }

            //ここまで

            Console.WriteLine(sum);
            Console.ReadKey();
        }

    }

    static class Func
    {
        static public long mod = 1000000007;
        static public void Swap<T>(ref T a, ref T b) { T temp = a; a = b; b = temp; }
        static public long GCD(long a, long b) { return b != 0 ? GCD(b, a % b) : a; }
        static public long LCM(long a, long b) { return (long)a * b / GCD(a, b); }
        static public long Factorial(long a) { return a == 1 ? a : a * Factorial(a - 1); }


        public static double ToAngle(this double radian)
        {
            return (double)(radian * 180 / Math.PI);
        }

        public static double ToRadian(this double angle)
        {
            return (double)(angle * Math.PI / 180);
        }

        static public long Conbination(long n, long m)
        {
            if (m == 0) return 1;
            if (n == 0) return 0;
            return n * Conbination(n - 1, m - 1) / m;
        }

        static public void Factoring(long num, ref long[] vs)
        {
            if (num == 1)
            {
                vs[1]++;
            }
            for (long i = 2; i < Math.Sqrt(num); i++)
            {
                if (num % i == 0)
                {
                    vs[i]++;
                    Factoring(num / i, ref vs);
                }
            }
            vs[1]++;
        }
    }

    static class IO
    {
        static public string ReadLineToString() { return Console.ReadLine(); }

        static public int ReadLineToInt() { return int.Parse(Console.ReadLine()); }
        static public long ReadLineToLong() { return long.Parse(Console.ReadLine()); }
        static public List<string> ReadLineToStringList() { return Console.ReadLine().Split(' ').ToList(); }

        static public List<int> ReadLineToIntList() { return Console.ReadLine().Split(' ').Select(int.Parse).ToList(); }
        static public List<long> ReadLineToLongList() { return Console.ReadLine().Split(' ').Select(long.Parse).ToList(); }

        static public List<double> ReadLineToDoubleList() { return Console.ReadLine().Split(' ').Select(double.Parse).ToList(); }

        static public void WriteStrings(object[] strings) { foreach (var o in strings) { Console.Write(o + " "); } }
    }
}