import java.io.InputStream;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashSet;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class Main {
    InputStream is;

    int __t__ = 1;
    int __f__ = 0;
    int __FILE_DEBUG_FLAG__ = __f__;
    String __DEBUG_FILE_NAME__ = "src/B1";

    FastScanner in;
    PrintWriter out;

    public void solve() {
        int n = in.nextInt();
        String s = in.next();
        long res = 0;
        int[] r = new int[n+1];
        int[] g = new int[n+1];
        int[] b = new int[n+1];
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) == 'G') {
                g[i+1] = 1;
            } else if (s.charAt(i) == 'R') {
                r[i+1] = 1;
            } else if (s.charAt(i) == 'B') {
                b[i+1] = 1;
            }
            r[i+1] += r[i];
            b[i+1] += b[i];
            g[i+1] += g[i];
        }

        HashSet<Character> set = new HashSet<>();
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                if (s.charAt(i) == s.charAt(j)) {
                    continue;
                }
                set.clear();
                set.add(s.charAt(i));
                set.add(s.charAt(j));
                int nj = j + j - i;
                if (!set.contains('R')) {
                    res += r[n] - r[j+1];

                    if (nj < n && s.charAt(nj) == 'R') {
                        res--;
                    }
                } else if (!set.contains('G')) {
                    res += g[n] - g[j+1];
                    if (nj < n && s.charAt(nj) == 'G') {
                        res--;
                    }
                } else {
                    res += b[n] - b[j+1];
                    if (nj < n && s.charAt(nj) == 'B') {
                        res--;
                    }
                }
            }
        }
        System.out.println(res);
    }

    long gcd(long a, long b) {
        if (b == 0) return a;
        return gcd(b, a % b);
    }

    public void run() {
        if (__FILE_DEBUG_FLAG__ == __t__) {
            try {
                is = new FileInputStream(__DEBUG_FILE_NAME__);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            System.out.println("FILE_INPUT!");
        } else {
            is = System.in;
        }
        in = new FastScanner(is);
        out = new PrintWriter(System.out);

        solve();
    }

    public static void main(final String[] args) {
        new Main().run();
    }
}


class FastScanner {
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public FastScanner(InputStream stream) {
        this.stream = stream;
        // stream = new FileInputStream(new File("dec.in"));
    }

    int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public boolean isEndline(int c) {
        return c == '\n' || c == '\r' || c == -1;
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public int[] nextIntArray(int n) {
        int[] array = new int[n];
        for (int i = 0; i < n; i++)
            array[i] = nextInt();

        return array;
    }

    public int[][] nextIntMap(int n, int m) {
        int[][] map = new int[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextIntArray(m);
        }
        return map;
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public long[] nextLongArray(int n) {
        long[] array = new long[n];
        for (int i = 0; i < n; i++)
            array[i] = nextLong();

        return array;
    }

    public long[][] nextLongMap(int n, int m) {
        long[][] map = new long[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextLongArray(m);
        }
        return map;
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }

    public double[] nextDoubleArray(int n) {
        double[] array = new double[n];
        for (int i = 0; i < n; i++)
            array[i] = nextDouble();

        return array;
    }

    public double[][] nextDoubleMap(int n, int m) {
        double[][] map = new double[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextDoubleArray(m);
        }
        return map;
    }

    public String next() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public String[] nextStringArray(int n) {
        String[] array = new String[n];
        for (int i = 0; i < n; i++)
            array[i] = next();

        return array;
    }

    public String nextLine() {
        int c = read();
        while (isEndline(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isEndline(c));
        return res.toString();
    }

    public int[][] nextPackedIntArrays(int packN, int size) {
        int[][] res = new int[packN][size];
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < packN; j++) {
                res[j][i] = nextInt();
            }
        }
        return res;
    }
}


Measured memory_bytes: 55652