#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]
#![allow(non_snake_case)]
#![allow(unused_mut)]
#![allow(unused_variables)]
#![allow(dead_code)]

type Vec2<T> = Vec<Vec<T>>;
type Vec3<T> = Vec<Vec<Vec<T>>>;

#[allow(unused_macros)]
macro_rules! invec {
    ( $ t : ty ) => {{
        let mut s = String::new();
        match std::io::stdin().read_line(&mut s) {
            Ok(0) => Vec::<$t>::new(),
            Ok(n) => s
                .trim()
                .split_whitespace()
                .map(|s| s.parse::<$t>().unwrap())
                .collect::<Vec<$t>>(),
            Err(_) => Vec::<$t>::new(),
        }
    }};
}

#[allow(unused_macros)]
macro_rules! get {
      ($t:ty) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.trim().parse::<$t>().unwrap()
          }
      };
      ($($t:ty),*) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              let mut iter = line.split_whitespace();
              (
                  $(iter.next().unwrap().parse::<$t>().unwrap(),)*
              )
          }
      };
      ($t:ty; $n:expr) => {
          (0..$n).map(|_|
              get!($t)
          ).collect::<Vec<_>>()
      };
      ($($t:ty),*; $n:expr) => {
          (0..$n).map(|_|
              get!($($t),*)
          ).collect::<Vec<_>>()
      };
      ($t:ty ;;) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.split_whitespace()
                  .map(|t| t.parse::<$t>().unwrap())
                  .collect::<Vec<_>>()
          }
      };
      ($t:ty ;; $n:expr) => {
          (0..$n).map(|_| get!($t ;;)).collect::<Vec<_>>()
      };
}

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($next:expr, [$t:tt]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    //($arg:expr) => (println!("{:?}",$arg));
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
      }
}
#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}

macro_rules! echo {
    ($($a:expr),*) => {
          $(println!("{}",$a))*
      }
}

use std::cmp::*;
use std::collections::*;
use std::ops::{Add, Div, Mul, Rem, Sub};

trait SafeRangeContain {
    fn safe_contains(&self, x: i64) -> bool;
}

impl SafeRangeContain for std::ops::Range<usize> {
    fn safe_contains(&self, x: i64) -> bool {
        if x < 0 {
            return false;
        }
        return self.contains(&(x as usize));
    }
}

#[allow(dead_code)]
static INF_I64: i64 = i64::max_value() / 2;
#[allow(dead_code)]
static INF_I32: i32 = i32::max_value() / 2;
#[allow(dead_code)]
static INF_USIZE: usize = usize::max_value() / 2;
#[allow(dead_code)]
static M_O_D: usize = 1000000007;
#[allow(dead_code)]
static PAI: f64 = 3.1415926535897932;

trait IteratorExt: Iterator {
    fn toVec(self) -> Vec<Self::Item>;
}

impl<T: Iterator> IteratorExt for T {
    fn toVec(self) -> Vec<Self::Item> {
        self.collect()
    }
}

trait CharExt {
    fn toNum(&self) -> usize;
    fn toAlphabetIndex(&self) -> usize;
    fn toNumIndex(&self) -> usize;
}
impl CharExt for char {
    fn toNum(&self) -> usize {
        return *self as usize;
    }
    fn toAlphabetIndex(&self) -> usize {
        return self.toNum() - 'a' as usize;
    }
    fn toNumIndex(&self) -> usize {
        return self.toNum() - '0' as usize;
    }
}

trait VectorExt {
    fn joinToString(&self, s: &str) -> String;
}
impl<T: ToString> VectorExt for Vec<T> {
    fn joinToString(&self, s: &str) -> String {
        return self
            .iter()
            .map(|x| x.to_string())
            .collect::<Vec<_>>()
            .join(s);
    }
}

trait StringExt {
    fn get_reverse(&self) -> String;
}
impl StringExt for String {
    fn get_reverse(&self) -> String {
        self.chars().rev().collect::<String>()
    }
}

trait UsizeExt {
    fn pow(&self, n: usize) -> usize;
}
impl UsizeExt for usize {
    fn pow(&self, n: usize) -> usize {
        return ((*self as u64).pow(n as u32)) as usize;
    }
}
//https://github.com/rust-lang-ja/ac-library-rs

pub mod fenwicktree {
    // Reference: https://en.wikipedia.org/wiki/Fenwick_tree
    #[derive(Clone, Debug)]
    pub struct FenwickTree<T> {
        n: usize,
        ary: Vec<T>,
        e: T,
    }

    impl<T: Clone + std::ops::AddAssign<T>> FenwickTree<T> {
        pub fn new(n: usize, e: T) -> Self {
            FenwickTree {
                n,
                ary: vec![e.clone(); n],
                e,
            }
        }
        pub fn accum(&self, mut idx: usize) -> T {
            let mut sum = self.e.clone();
            while idx > 0 {
                sum += self.ary[idx - 1].clone();
                idx &= idx - 1;
            }
            sum
        }
        /// performs data[idx] += val;
        pub fn add<U: Clone>(&mut self, mut idx: usize, val: U)
        where
            T: std::ops::AddAssign<U>,
        {
            let n = self.n;
            idx += 1;
            while idx <= n {
                self.ary[idx - 1] += val.clone();
                idx += idx & idx.wrapping_neg();
            }
        }
        /// Returns data[l] + ... + data[r - 1].
        pub fn sum(&self, l: usize, r: usize) -> T
        where
            T: std::ops::Sub<Output = T>,
        {
            self.accum(r) - self.accum(l)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn fenwick_tree_works() {
            let mut bit = FenwickTree::new(5, 0i64);
            // [1, 2, 3, 4, 5]
            for i in 0..5 {
                bit.add(i, i as i64 + 1);
            }
            assert_eq!(bit.sum(0, 5), 15);
            assert_eq!(bit.sum(0, 4), 10);
            assert_eq!(bit.sum(1, 3), 5);
        }
    }
}
use fenwicktree::*;

fn main() {
    solve();
}

fn solve() {
    let mut ans: u64 = 0;
    let N = get!(usize);
    let s = get!(String)
        .chars()
        .map(|x| {
            x.to_lowercase()
                .to_string()
                .chars()
                .next()
                .unwrap()
                .toAlphabetIndex()
        })
        .toVec();

    mydbg!(s);
    let mut a = vec![FenwickTree::new(N, 0_i32); 26];
    for i in 0..N {
        let t = s[i];
        a[t].add(i, 1);
    }
    let mut agct = ['a', 'g', 'c', 't']
        .iter()
        .map(|&x| x.toAlphabetIndex())
        .toVec();
    let mut L = 0;
    let mut count = FenwickTree::new(N, 0_i32);
    for i in 0..N {
        count.add(i, 1);
    }
    loop {
        let mut c1 = 0;
        for &item in &agct {
            c1 += a[item].accum(N);
        }

        if c1 == 0 {
            break;
        }

        let mut l = -1;
        let mut r = N as i32;
        while l + 1 != r {
            let m = (l + r) / 2;

            if count.accum((m + 1) as usize) < c1 {
                l = m;
            } else {
                r = m;
            }
        }
        let mut c1_index = r as usize;

        let mut c2 = 0;
        for i in 0..26 {
            let k = a[i].accum(c1_index + 1) - a[i].accum(c1_index);
            if k == 1 {
                a[i].add(c1_index, -1);
                c2 = a[i].accum(N) as usize;
                count.add(c1_index, -1);
                break;
            }
        }

        for i in 0..4 {
            agct[i] += 26;
            agct[i] -= c2;
            agct[i] %= 26;
        }
        ans += 1;
    }

    echo!(ans);
}