#include <algorithm>
#include <cstdio>
#include <cstring>
using namespace std;
int n, a[50001][2];
int seq[50001], w[50001];
int main()
{
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) scanf("%d%d", &a[i][0], &a[i][1]);
    int x = 0, ans1, ans2;
    for (int i = 1; i <= n; i++) x ^= a[i][0], x ^= a[i][1];
    if (x)
        printf("-1");
    else
    {
        seq[0] = 1, seq[1] = a[1][0];
        for (int i = 2; i < n; i++) seq[i] = a[seq[i - 1]][seq[i - 2] == a[seq[i - 1]][0]];
        if (seq[n - 1] != a[1][1])
            printf("-1");
        else
        {
            memset(w, 0, sizeof(w));
            for (int i = 0; i < n; i++)
                w[(seq[i] - i + n) % n]++;
            ans1 = *max_element(w, w + n);
            reverse(seq, seq + n);
            memset(w, 0, sizeof(w));
            for (int i = 0; i < n; i++)
                w[(seq[i] - i + n) % n]++;
            ans2 = *max_element(w, w + n);
            printf("%d", n - max(ans1, ans2));
        }
    }
    return 0;
}