#include <bits/stdc++.h>
using namespace std;
const int N = 110;
int n, d[N][N], ans;
int main() {
    scanf("%d", &n);

    int u, v;
    for (int i = 1; i < n; i++) scanf("%d%d", &u, &v), d[u][v] = 1;

    // Floyd求传递闭包
    for (int k = 1; k <= n; k++)
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= n; j++)
                d[i][j] |= d[i][k] & d[k][j];

    for (int i = 1; i <= n; i++) {
        int cnt = 0;
        for (int j = 1; j <= n; j++)
            if (d[j][i]) cnt++; //有多少个点可以到达i点

        if (cnt == n - 1) {
            ans = i;
            break;
        }
    }
    if (ans)
        printf("%d", ans);
    else
        puts("-1");
    return 0;
}