/*
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//*/
 
#include <bits/stdc++.h>
 
// #include <atcoder/all>
 
using namespace std;
// using namespace atcoder;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(int)(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
void YES(bool t=true) {cout<<(t?"YES":"NO")<<endl;}
void Yes(bool t=true) {cout<<(t?"Yes":"No")<<endl;}
void yes(bool t=true) {cout<<(t?"yes":"no")<<endl;}
void NO(bool t=true) {cout<<(t?"NO":"YES")<<endl;}
void No(bool t=true) {cout<<(t?"No":"Yes")<<endl;}
void no(bool t=true) {cout<<(t?"no":"yes")<<endl;}
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
int popcount(ll t) { return __builtin_popcountll(t); }
// int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
// int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {0, 1, 0, -1}, dy = {-1, 0, 1, 0};
// vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
struct Setup_io {
    Setup_io() {
        ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        cout << fixed << setprecision(25);
    }
} setup_io;
// const ll MOD = 1000000007;
const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'



signed main() {
    int n;
    cin >> n;
    vi a(n), b(n);
    vi pos(2 * n);
    rep (i, n) {
        cin >> a[i] >> b[i];
        a[i]--; b[i]--;
        pos[a[i]] = pos[b[i]] = i;
    }

    int l = 0, r = 2 * n - 1;
    int done = 0;
    int ans = 0;
    vector<bool> used(n);
    while (done < n) {
        int omo = 0, ura = 0;

        vector<pii> ue, sita;
        queue<pii> qu;
        qu.push(pii(l, 1));
        l++;
        while (qu.size()) {
            pii temp = qu.front();
            qu.pop();
            bool t = temp.second;
            int u = temp.first;
            int i = pos[u];
            if (used[i]) continue;
            used[i] = true;

            if (t) {
                pii p;
                int ai;
                if (a[i] == u) {
                    omo++;
                    ai = b[i];
                    p = pii(a[i], b[i]);
                }
                else {
                    ura++;
                    ai = a[i];
                    p = pii(b[i], a[i]);
                }

                ue.push_back(p);
                while (r >= ai) {
                    qu.push(pii(r, 0));
                    r--;
                }
            }
            else {
                pii p;
                int ai;
                if (a[i] == u) {
                    omo++;
                    ai = b[i];
                    p = pii(a[i], b[i]);
                }
                else {
                    ura++;
                    ai = a[i];
                    p = pii(b[i], a[i]);
                }

                sita.push_back(p);
                while (l <= ai) {
                    qu.push(pii(l, 1));
                    l++;
                }
            }
        }
        // DEBUG_VEC(ue);
        // DEBUG_VEC(sita);

        while (sita.size()) {
            pii temp = sita.back();
            ue.push_back(temp);
            sita.pop_back();
        }

        for (int i = 0; i < (int)ue.size() - 1; i++) {
            if (ue[i].first > ue[i + 1].first) {
                cout << -1 << endl;
                return 0;
            }
            if (ue[i].second < ue[i + 1].second) {
                cout << -1 << endl;
                return 0;
            }
        }

        ans += min(omo, ura);
        done += omo + ura;
    }
    cout << ans << endl;
}