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.RegularExpressions;
using static System.Math;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            A();
        }

        static void A()
        {
            var N = ReadInt();
            var S = Read();
            Println(Regex.Matches(S, "ABC").Count);
        }

        static List<long> ReadLines(long n)
        {
            var l = new List<long>();
            for (long i = 0; i < n; i++)
            {
                l.Add(ReadLong());
            }
            return l;
        }

        static void YESOrNO(bool e) => Println(e ? "YES" : "NO");

        static void YesOrNo(bool e) => Println(e ? "Yes" : "No");

        static IEnumerable<int> Range(int start, int stop)
        {
            if (start < 0 || stop < 0 || start > stop || (start <= 0 && stop <= 0))
                return new List<int>();

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

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

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

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

        static long Gcd(long a, long b) => b == 0 ? a : Gcd(b, a % b);
        static long Lcm(long a, long b) => a / Gcd(a, b) * b;

        static bool IsPrime(int x)
        {
            if (x <= 1 || (x != 2 && x % 2 == 0)) return false;
            if (x == 2) return true;

            for (int i = 3; i < x; i += 2)
                if (x % i == 0) return false;

            return true;
        }

        static string Read() => Console.ReadLine();
        static int ReadInt() => int.Parse(Read());
        static long ReadLong() => long.Parse(Read());

        static List<string> ReadSplit() => Read().Split().ToList();
        static List<int> ReadSplitInt() => Read().Split().Select(int.Parse).ToList();
        static List<long> ReadSplitLong() => Read().Split().Select(long.Parse).ToList();

        static void Print(object value) => Console.Write(value.ToString());
        static void Println(object value) => Console.WriteLine(value.ToString());
        static string Join<T>(IEnumerable<T> list) => string.Join("", list);
    }

    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);
        }
    }
}