def maxPathValue(n, m, edges, values):
    adj = [[] for i in range(n+1)]

    inDegree = [0 for i in range(n+1)]

    for i in range(m):
        adj[edges[i][0]].append(edges[i][1])
        inDegree[edges[i][1]] += 1

    q = []

    dp = [[0 for i in range(26)] for j in range(n + 1)]

    for i in range(1, n + 1):
        if (inDegree[i] == 0):
            q.append(i)

    popCounts = 0
    ans = 0

    while (len(q) > 0):
        curr = q.pop(0)
        popCounts += 1

        num = ord(values[curr - 1]) - 97
        dp[curr][num] += 1
        ans = max(ans, dp[curr][num])

        for i in adj[curr]:

            for j in range(26):
                dp[i][j] = max(dp[i][j], dp[curr][j])

            inDegree[i] -= 1

            if (inDegree[i] == 0):
                q.append(i)

        if (popCounts == n):
            break


    if (len(q) > 0 or popCounts < n):
        return -1

    return ans