fn solve() {
    let stdin = std::io::stdin();
    let mut scn = Scanner { stdin: stdin.lock() };
    let n: usize = scn.read();
    let s = scn.chars();
    let mut rgb = vec![(0i64, 0i64, 0i64); n];
    for (idx, &c) in s.iter().enumerate().rev() {
        if c == 'R' {
            rgb[idx].0 += 1;
        } else if c == 'G' {
            rgb[idx].1 += 1;
        } else if c == 'B' {
            rgb[idx].2 += 1;
        }
        if idx + 1 < n {
            let (r, g, b) = rgb[idx + 1];
            rgb[idx].0 += r;
            rgb[idx].1 += g;
            rgb[idx].2 += b;
        }
    }
    let mut result: i64 = 0;
    for i in 0..n {
        for j in i + 1..n {
            if s[i] == s[j] {
                continue;
            }
            let k = j + j - i;
            /*if k < n {
                if s[k] == s[i] || s[k] == s[j] {
                    continue;
                }
            }*/
            let colors = vec!['R', 'G', 'B'];
            let c: Vec<char> = colors.into_iter().filter(|&x| x != s[i] && x != s[j]).collect();
            let c = *c.last().unwrap();
            if c == 'R' {
                result += rgb[j].0;
            } else if c == 'G' {
                result += rgb[j].1;
            } else if c == 'B' {
                result += rgb[j].2;
            }
            if k < n {
                if s[k] == c {
                    result -= 1;
                }
            }
        }
    }
    println!("{}", result);
}

fn main() {
    std::thread::Builder::new()
        .stack_size(64 * 1024 * 1024) // 64MB
        .spawn(|| solve())
        .unwrap()
        .join()
        .unwrap();
}
pub mod utils {
    const DYX: [(isize, isize); 8] = [
        (0, 1),   //right
        (1, 0),   //down
        (0, -1),  //left
        (-1, 0),  //top
        (1, 1),   //down right
        (-1, 1),  //top right
        (1, -1),  //down left
        (-1, -1), //top left
    ];

    pub fn try_adj(y: usize, x: usize, dir: usize, h: usize, w: usize) -> Option<(usize, usize)> {
        let ny = y as isize + DYX[dir].0;
        let nx = x as isize + DYX[dir].1;
        if ny >= 0 && nx >= 0 {
            let ny = ny as usize;
            let nx = nx as usize;
            if ny < h && nx < w {
                Some((ny, nx))
            } else {
                None
            }
        } else {
            None
        }
    }
}

//snippet from kenkoooo
pub struct Scanner<R> {
    stdin: R,
}

impl<R: std::io::Read> Scanner<R> {
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .stdin
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn read_line(&mut self) -> String {
        use std::io::Read;
        let buf = self
            .stdin
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b'\n' || b == b'\r')
            .take_while(|&b| b != b'\n' && b != b'\r')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }

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


Measured memory_bytes: 2312