#![allow(non_snake_case)]
#![allow(dead_code)]
#![allow(unused_macros)]
#![allow(unused_imports)]

use std::str::FromStr;
use std::io::*;
use std::collections::*;
use std::cmp::*;

struct Scanner<I: Iterator<Item = char>> {
    iter: std::iter::Peekable<I>,
}

macro_rules! exit {
    () => {{
        exit!(0)
    }};
    ($code:expr) => {{
        if cfg!(local) {
            writeln!(std::io::stderr(), "===== Terminated =====")
                .expect("failed printing to stderr");
        }
        std::process::exit($code);
    }}
}

impl<I: Iterator<Item = char>> Scanner<I> {
    pub fn new(iter: I) -> Scanner<I> {
        Scanner {
            iter: iter.peekable(),
        }
    }

    pub fn safe_get_token(&mut self) -> Option<String> {
        let token = self.iter
            .by_ref()
            .skip_while(|c| c.is_whitespace())
            .take_while(|c| !c.is_whitespace())
            .collect::<String>();
        if token.is_empty() {
            None
        } else {
            Some(token)
        }
    }

    pub fn token(&mut self) -> String {
        self.safe_get_token().unwrap_or_else(|| exit!())
    }

    pub fn get<T: FromStr>(&mut self) -> T {
        self.token().parse::<T>().unwrap_or_else(|_| exit!())
    }

    pub fn vec<T: FromStr>(&mut self, len: usize) -> Vec<T> {
        (0..len).map(|_| self.get()).collect()
    }

    pub fn mat<T: FromStr>(&mut self, row: usize, col: usize) -> Vec<Vec<T>> {
        (0..row).map(|_| self.vec(col)).collect()
    }

    pub fn char(&mut self) -> char {
        self.iter.next().unwrap_or_else(|| exit!())
    }

    pub fn chars(&mut self) -> Vec<char> {
        self.get::<String>().chars().collect()
    }

    pub fn mat_chars(&mut self, row: usize) -> Vec<Vec<char>> {
        (0..row).map(|_| self.chars()).collect()
    }

    pub fn line(&mut self) -> String {
        if self.peek().is_some() {
            self.iter
                .by_ref()
                .take_while(|&c| !(c == '\n' || c == '\r'))
                .collect::<String>()
        } else {
            exit!();
        }
    }

    pub fn peek(&mut self) -> Option<&char> {
        self.iter.peek()
    }
}

fn main() {
    let cin = stdin();
    let cin = cin.lock();
    let mut sc = Scanner::new(cin.bytes().map(|c| c.unwrap() as char));
    let N: usize = sc.get();
    let S: Vec<char> = sc.chars();
    let mut cnt_r = vec![0; N];
    let mut cnt_g = vec![0; N];
    let mut cnt_b = vec![0; N];
    for i in 0..N {
        if S[i] == 'R' {
            cnt_r[i] += 1;
        }
        if S[i] == 'G' {
            cnt_g[i] += 1;
        }
        if S[i] == 'B' {
            cnt_b[i] += 1;
        }
    }
    let mut cum_r = vec![0; N+1];
    let mut cum_g = vec![0; N+1];
    let mut cum_b = vec![0; N+1];
    for i in 0..N {
        cum_r[i+1] = cum_r[i] + cnt_r[i];
        cum_g[i+1] = cum_g[i] + cnt_g[i];
        cum_b[i+1] = cum_b[i] + cnt_b[i];
    }
    let mut ans: usize = 0;
    for i in 0..N-2 {
        for j in i+1..N-1 {
            if S[i] == S[j] {
                continue;
            }
            let a = min(S[i], S[j]);
            let b = max(S[i], S[j]);
            if a == 'B' && b == 'G' {
                ans += cum_r[N] - cum_r[j+1];
                if 2 * j - i < N && S[2 * j - i] == 'R' {
                    ans -= 1;
                }
            }
            if a == 'B' && b == 'R' {
                ans += cum_g[N] - cum_g[j+1];
                if 2 * j - i < N && S[2 * j - i] == 'G' {
                    ans -= 1;
                }
            }
            if a == 'G' && b == 'R' {
                ans += cum_b[N] - cum_b[j+1];
                if 2 * j - i < N && S[2 * j - i] == 'B' {
                    ans -= 1;
                }
            }
        }
    }
    println!("{}", ans);
}


Measured memory_bytes: 2232