using System;

namespace AtCoder.Problems
{
    public class ProblemD
    {
        public static void Main(string[] args)
        {
            var s = new SolveD();
            s.Solve();
        }

        public class Scanner
        {
            private string[] _s;
            private int _i;
            private readonly char[] _separators = new char[] { ' ' };

            public Scanner()
            {
                _s = new string[0];
                _i = 0;
            }

            public string Next()
            {
                if (_i < _s.Length) return _s[_i++];
                string st = Console.ReadLine();
                while (st == "") st = Console.ReadLine();
                _s = st.Split(_separators, StringSplitOptions.RemoveEmptyEntries);
                if (_s.Length == 0) return Next();
                _i = 0;
                return _s[_i++];
            }

            public int NextInt() => int.Parse(Next());
            public long NextLong() => long.Parse(Next());
            public ulong NextULong() => ulong.Parse(Next());
            public double NextDouble() => double.Parse(Next());

            public int[] GetIntArray(int n, int add = 0)
            {
                int[] array = new int[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextInt() + add;
                }
                return array;
            }

            public long[] GetLongArray(int n, long add = 0)
            {
                long[] array = new long[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextLong() + add;
                }
                return array;
            }

            public double[] GetDoubleArray(int n, double add = 0)
            {
                double[] array = new double[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextDouble() + add;
                }
                return array;
            }
        }
    }

    public class SolveD
    {
        public void Solve()
        {
            var cin = new ProblemD.Scanner();
            var n = cin.NextInt();
            var s = cin.Next();

            var result = GetAnswer(n, s);
            Console.WriteLine(result);
        }

        public long GetAnswer(int n, string s)
        {
            var dpR = new int[n + 1];
            var dpG = new int[n + 1];
            var dpB = new int[n + 1];


            for (int i = n - 1; i >= 0; i--)
            {
                dpR[i] = s[i] == 'R' ? dpR[i + 1] + 1 : dpR[i + 1];
                dpG[i] = s[i] == 'G' ? dpG[i + 1] + 1 : dpG[i + 1];
                dpB[i] = s[i] == 'B' ? dpB[i + 1] + 1 : dpB[i + 1];
            }

            long sum = 0;
            for (int i = 0; i < n - 2; i++)
            {
                for (int j = i + 1; j < n - 1; j++)
                {
                    if (s[i] == s[j]) { continue; }

                    var c = 'R';
                    if (s[i] == c || s[j] == c) { c = 'G'; }
                    if (s[i] == c || s[j] == c) { c = 'B'; }

                    switch (c)
                    {
                        case 'R':
                            sum += dpR[j + 1];
                            break;
                        case 'G':
                            sum += dpG[j + 1];
                            break;
                        case 'B':
                            sum += dpB[j + 1];
                            break;
                    }

                    var disIndex = (j - i) + j;
                    if (disIndex < n && s[disIndex] == c)
                    {
                        sum--;
                    }
                }
            }
            return sum;
        }
    }
}


Measured memory_bytes: 27608