RESEARCH

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;

template <class T>
void read(T &x)
{
    x = 0;
    register int c;
    while ((c = getchar()) && (c > '9' || c < '0'))
        ;
    for (; c >= '0' && c <= '9'; c = getchar())
        x = x * 10 + c - '0';
}

constexpr bool typetest = 0;
constexpr int N = 1e5 + 5;
int n, m, l(0), com[N];
bool ck[N];
string s;
int x[N];
vector<int> adj[N], nadj[N], ss;

void Read()
{
    cin >> n >> m >> s;
    s = " " + s;
    for (int i = 1; i <= m; ++i)
    {
        int u, v;
        cin >> u >> v;
        adj[u].emplace_back(v);
        nadj[v].emplace_back(u);
    }
}

void dfs(int v)
{
    ck[v] = 1;
    for (auto i : adj[v])
        if (!ck[i])
            dfs(i);
    ss.emplace_back(v);
}

void dfs_com(int v)
{
    com[v] = l;
    for (auto i : nadj[v])
        if (!com[i])
            dfs_com(i);
}

void Kosaraju()
{
    for (int i = 1; i <= n; ++i)
        if (!ck[i])
            dfs(i);

    while (!ss.empty())
    {
        int c = ss.back();
        ss.pop_back();
        if (com[c])
        {
            cout << -1;
            exit(0);
        }
        ++l;
        dfs_com(c);
    }
}

int Cal(int v)
{
    queue<int> q[2];

    for (int i = 1; i <= n; ++i)
        if ((x[i] = nadj[i].size()) == 0)
        {
            if (s[i] == 'C')
                q[v].emplace(i);
            else
                q[s[i] - 'A'].emplace(i);
        }

    int ans(0);

    for (int turn = v; !q[turn].empty(); turn ^= 1)
    {
        ++ans;
        while (!q[turn].empty())
        {
            int c = q[turn].front();
            q[turn].pop();

            for (auto i : adj[c])
            {
                --x[i];
                if (x[i] == 0)
                {
                    if (s[i] == 'C')
                        q[turn].emplace(i);
                    else
                        q[s[i] - 'A'].emplace(i);
                }
            }
        }
    }

    return ans - 1;
}

void Solve()
{
    Kosaraju();
    cout << Cal(0);
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int t(1);
    if (typetest)
        cin >> t;
    for (int _ = 1; _ <= t; ++_)
    {
        //cout << "Case #" << _ << "\n";
        Read();
        Solve();
    }
}