import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Spandan Mishra
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        DRGBTriplets solver = new DRGBTriplets();
        solver.solve(1, in, out);
        out.close();
    }

    static class DRGBTriplets {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.readInt();
            char[] s = in.readString().toCharArray();

            long r = 0, g = 0, b = 0;
            for (int i = 0; i < s.length; i++) {
                if (s[i] == 'R')
                    r++;
                else if (s[i] == 'B')
                    b++;
                else
                    g++;
            }

            long res = r * g * b;

            for (int i = 0; i < n - 2; i++) {
                for (int j = i + 1; j < n; j++) {
                    int k = 2 * j - i;

                    if (k >= n)
                        break;

                    if (s[i] != s[j] && s[i] != s[k] && s[j] != s[k])
                        res--;
                }
            }

            System.out.println(res);

        }

    }

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

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

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

        public String readString() {
            final StringBuilder stringBuilder = new StringBuilder();
            int c = read();
            while (isSpaceChar(c))
                c = read();
            do {
                stringBuilder.append((char) c);
                c = read();
            } while (!isSpaceChar(c));
            return stringBuilder.toString();
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

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

    }
}



Measured memory_bytes: 32648