#include <iostream>
#include <vector>
using namespace std;
int main ()
{
    int n, m;
    cin >> n >> m;
    int p[n]; bool used[n];

    for (int i = 0; i < n; i++)
    {
        used[i] = false;
    }

    vector <vector <int> > g(m);

    for (int i = 0, x; i < n - 1; i++)
    {
        cin >> x;
        p[x] = i;
    }

    int s;
    cin >> s;

    for (int i = 0, a, b; i < n; i++)
    {
        cin >> a >> b;
        g[a - 1].push_back(b - 1);
        g[b - 1].push_back(a - 1);
    }

    vector <int> v;
    v.push_back(s);
    int ans = n - 1;
    used[s] = true;

    while (!v.empty())
    {
        int top = v[v.size() - 1];
        v.pop_back();
        for (int i = 0; i < g[top].size(); i++)
        {
            if (!used[g[top][i]])
            {
                g[top][i] = true;
                v.push_back(g[top][i]);
                if (p[g[top][i]] < ans) ans = p[g[top][i]];
            }
        }
    }

    cout << ans;

    return 0;
}