#include <iostream>
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <vector>
#include <queue>
#include <stdlib.h>
#include <time.h>
#include <map>
#include <complex>
#include <math.h>
#include <stack>
#include <unordered_map>
#include <set>
#include <string.h>
#include <iomanip>
#include <bitset>
#include <random>
using namespace std;
//#define int long long
#define P make_pair
#define A first
#define B second
typedef pair<int, int> pii;
int fa[200010];
priority_queue<pii> q[200010];
vector<pii> t[200010];
signed main() {
    ios::sync_with_stdio(0);
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)cin >> fa[i];
    int m, ok = 1;
    cin >> m;
    for (int i = 1; i <= m; i++) {
        int a, b;
        cin >> a >> b;
        int c = a, d = 0;
        while (1) {
            if (c == b)break;
            c = fa[c];
            d++;
            if (d == n)break;
        }
        if (d == n)ok = 0;
        else q[a].push(P(d, b));
    }
    if (ok == 0) {
        cout << -1 << endl;
        return 0;
    }
    int ans = 0;
    while (1) {
        ok = 1;
        for (int i = 1; i <= n; i++) {
            if (!q[i].empty() && q[i].top().A != 0)ok = 0, t[fa[i]].push_back(P(q[i].top().A - 1, q[i].top().B)), q[i].pop();
        }
        for (int i = 1; i <= n; i++)while (!t[i].empty())q[i].push(t[i][t[i].size() - 1]), t[i].pop_back();
        if (!ok)ans++;
        else {
            cout << ans << endl;
            return 0;
        }
    }
    return 0;
}