#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

int main(){
    int N; cin >> N;

    int MAX = 200000;
    vector<int> c(MAX+1, 0), d(MAX+1, 0);

    rep(i, N){
        int H, W; cin >> H >> W;
        c[H]++, d[W]++;
    }

    int id1 = -1, id2 = -1;
    int cnt1 = 0, cnt2 = 0;
    int K = 0;

    rep(i, MAX+1){
        if(c[i] > d[i]){
            cnt1 += c[i]-d[i];
            id1 = i;
        }
        if(c[i] < d[i]){
            cnt2 += d[i]-c[i];
            id2 = i;
        }
        if(c[i] > 0) K++;
    }

    //cout << cnt1 << '\n';

    if(cnt1 > 1) {cout << "-1\n"; return 0;}

    if(id1 == -1) cout << K << '\n';
    else cout << "1\n";
}