import sys, os, io
input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline
def topological_sort():
q, k = [], 0
cnt = [0] * (n + 1)
for i in range(1, n + 1):
for j in G[i]:
cnt[j] += 1
for i in range(1, n + 1):
if not cnt[i]:
q.append(i)
while len(q) ^ k:
i = q[k]
for j in G[i]:
cnt[j] -= 1
if not cnt[j]:
q.append(j)
k += 1
return q
def f(u, v):
return u * 26 + v
n, m = map(int, input().split())
s = [0] + list(input().rstrip())
G = [[] for _ in range(n + 1)]
for _ in range(m):
x, y = map(int, input().split())
G[x].append(y)
g = topological_sort()
if len(g) ^ n:
ans = -1
print(ans)
exit()
dp = [0] * (26 * (n + 1))
for i in g:
dp[f(i, s[i] - 97)] += 1
for j in range(26):
dp0 = dp[f(i, j)]
for k in G[i]:
u = f(k, j)
dp[u] = max(dp[u], dp0)
ans = max(dp)
print(ans)#include <bits/stdc++.h>
#define N 300010
using namespace std;
struct Edge{
int from, to, next;
} edge[N];
int head[N], tot;
inline void addedge(int u, int v){
edge[++tot] = (Edge){u, v, head[u]}, head[u] = tot;
}
int f[N][26], n, m, d[N];
char s[N];
queue<int> Q;
int main(){
scanf("%d%d", &n, &m);
scanf("%s", s + 1);
for (int i = 1; i <= m; i++){
int x, y;
scanf("%d%d", &x, &y);
addedge(x, y);
d[y] ++;
}
for (int i = 1; i <= n; i++){
if (!d[i]){
Q.push(i);
f[i][s[i] - 'a'] = 1;
}
}
int rem = n;
while (!Q.empty()){
int now = Q.front();
Q.pop();
rem --;
for (int i = head[now]; i; i = edge[i].next){
Edge e = edge[i];
for (int j = 0; j < 26; j++){
f[e.to][j] = max(f[e.to][j], f[now][j] + (s[e.to] - 'a' == j));
}
d[e.to] --;
if (!d[e.to]) Q.push(e.to);
}
}
if (rem) return puts("-1"), 0;
int ans = 0;
for (int i = 1; i <= n; i++){
for (int j = 0; j < 26; j++){
ans = max(ans, f[i][j]);
}
}
printf("%d\n", ans);
return 0;
}1674E - Breaking the Wall | 1282A - Temporarily unavailable |
1366C - Palindromic Paths | 336A - Vasily the Bear and Triangle |
926A - 2-3-numbers | 276D - Little Girl and Maximum XOR |
1253C - Sweets Eating | 1047A - Little C Loves 3 I |
758D - Ability To Convert | 733A - Grasshopper And the String |
216A - Tiling with Hexagons | 1351B - Square |
1225A - Forgetting Things | 1717A - Madoka and Strange Thoughts |
1717B - Madoka and Underground Competitions | 61B - Hard Work |
959B - Mahmoud and Ehab and the message | 802G - Fake News (easy) |
1717C - Madoka and Formal Statement | 420A - Start Up |
1031A - Golden Plate | 1559C - Mocha and Hiking |
427B - Prison Transfer | 330A - Cakeminator |
426A - Sereja and Mugs | 363A - Soroban |
1585C - Minimize Distance | 1506E - Restoring the Permutation |
1539A - Contest Start | 363D - Renting Bikes |