// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let 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)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------

fn run() {
    input! {
        n: usize,
        s: bytes,
        e: [(usize1, usize1, bytes); n - 1],
    }
    let mut g = vec![vec![]; n];
    for (a, b, c) in e {
        let c = c[0];
        g[a].push((b, c));
        g[b].push((a, c));
    }
    let mut topo = vec![0];
    for i in 0..n {
        let v = topo[i];
        for (u, _) in g[v].clone() {
            let x = g[u].iter().position(|p| p.0 == v).unwrap();
            g[u].remove(x);
            topo.push(u);
        }
    }
    let mut ans = 0;
    let w = s.len();
    let mut up = vec![vec![]; n];
    let mut down = vec![vec![]; n];
    for &v in topo.iter().rev() {
        if g[v].is_empty() {
            up[v] = vec![0; w + 1];
            down[v] = vec![0; w + 1];
            continue;
        }
        for _ in 0..2 {
            let mut a = vec![0; w + 1];
            let mut memo = vec![vec![0; w + 1]];
            for &(u, c) in g[v].iter() {
                for (i, &s) in s.iter().enumerate().rev() {
                    if s == c {
                        a[i] = a[i].max(a[i + 1]).max(down[u][i]).max(down[u][i + 1] + 1);
                    } else {
                        a[i] = a[i].max(a[i + 1]).max(down[u][i]);
                    }
                }
                memo.push(a.clone());
            }
            down[v] = memo.last().unwrap().clone();
            let mut b = vec![0; w + 1];
            for &(u, c) in g[v].iter().rev() {
                memo.pop();
                let tl = memo.last().unwrap();
                for (i, &s) in s.iter().enumerate() {
                    if s == c {
                        b[i + 1] = b[i + 1].max(b[i]).max(up[u][i + 1]).max(up[u][i] + 1);
                    } else {
                        b[i + 1] = b[i + 1].max(b[i]).max(up[u][i + 1]);
                    }
                }
                ans = ans.max(b.iter().zip(tl.iter()).map(|p| *p.0 + *p.1).max().unwrap());
            }
            up[v] = b.clone();
            g[v].reverse();
        }
    }
    ans = ans.max(*down.iter().flatten().chain(up.iter().flatten()).max().unwrap());
    println!("{}", ans);
}

fn main() {
    run();
}