human
Write me a Python code to find a path whose value is the largest given the following:
Solve in Python:
You are given a graph with $n$ nodes and $m$ directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is $3$. Your task is find a path whose value is the largest.

-----Input-----

The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), denoting that the graph has $n$ nodes and $m$ directed edges.

The second line contains a string $s$ with only lowercase English letters. The $i$-th character is the letter assigned to the $i$-th node.

Then $m$ lines follow. Each line contains two integers $x, y$ ($1 \leq x, y \leq n$), describing a directed edge from $x$ to $y$. Note that $x$ can be equal to $y$ and there can be multiple edges between $x$ and $y$. Also the graph can be not connected.

-----Output-----

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

-----Examples-----
Input
5 4
abaca
1 2
1 3
3 4
4 5

Output
3

Input
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4

Output
-1

Input
10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7

Output
4


-----Note-----

In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
assistant
#Here is Python code to find a path whose value is the largest.

# NOT MY CODE
# https://codeforces.com/contest/919/submission/80857731
 
from types import GeneratorType
 
def bootstrap(f, stack=[]):
    def wrappedfunc(*args, **kwargs):
        to = f(*args, **kwargs)
        if stack:
            return to
        else:
            while True:
                if type(to) is GeneratorType:
                    stack.append(to)
                    to = next(to)
                else:
                    stack.pop()
                    if not stack:
                        return to
                    to = stack[-1].send(to)
    return wrappedfunc
 
import sys
input=sys.stdin.readline
from collections import defaultdict as dd
'''
def iscyclic(g):
    for i in range(1,n+1):
        if(
'''
n,m=list(map(int,input().split()))
s=input()
d=dd(list)
lol=0
for i in range(m):
    u,v=list(map(int,input().split()))
    if(u==v):
        lol=1
        print(-1)
        return
    d[u].append(v)
'''
if(iscyclic(d)):
    lol=1
    print(-1)
    return
'''
vis=[0]*(n+1)
rvis=[0]*(n+1)
cou=[0]
dp=[dd(int) for i in range(n+1)]
mx=[0]
h=[0]
#print(d)
@bootstrap
def dfs(u):
    if(h[0]==1):
        yield
    vis[u]=1
    rvis[u]=1
    for i in d[u]:
        if(h[0]==1):
            yield
        if(vis[i]==0):
            yield dfs(i)
            if(h[0]==1):
                yield            
        elif(rvis[i]==True):
            h[0]=1
            yield
    rvis[u]=0
    if(h[0]==1):
        yield
    for i in d[u]:
        for j in dp[i]:
            dp[u][j]=max(dp[u][j],dp[i][j])
            mx[0]=max(mx[0],dp[u][j])
    dp[u][s[u-1]]+=1
    #print(u,dp[u])
    mx[0]=max(mx[0],dp[u][s[u-1]])
    yield
#print(dp)
for i in range(1,n+1):
    if(vis[i]==0):
        cou=[0]
        dfs(i)
if(h[0]):
    print(-1)
else:
    print(mx[0])