//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using ll = int64_t;
using ull = uint64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T>>;
const int INF = 0xccccccc;
const ll LINF = 0xcccccccccccccccLL;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

#undef _GLIBCXX_DEBUG
string to_string(const string &s) {return '"' + s + '"';}
string to_string(const char *s) {return to_string(string(s));}
string to_string(bool b) {return b?"true":"false";}
string to_string(vector<bool> v) {
  string res = "{";
  for(int i = 0; i < int(v.size()); i++) {
    if(i) res += ", ";
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template<size_t N>
string to_string(bitset<N> v) {
  string res;
  for(size_t i = 0; i < N; i++) res += char('0' + v[i]);
  return res;
}
template<class A, class B>
string to_string(pair<A, B>);
template<class A, class B, class C>
string to_string(tuple<A, B, C>);
template<class A, class B, class C, class D>
string to_string(tuple<A, B, C, D>);
template<class A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for(const auto &x:v) {
    if(!first) res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template<class A, class B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template<class A, class B, class C>
string to_string(tuple<A, B, C> t) {
  return "(" + to_string(get<0>(t)) + ", " + to_string(get<1>(t)) + ", " + to_string(get<2>(t)) + ")";
}
template<class A, class B, class C, class D>
string to_string(tuple<A, B, C, D> t) {
  return "(" + to_string(get<0>(t)) + ", " + to_string(get<1>(t)) + ", " + to_string(get<2>(t)) + ", " + to_string(get<3>(t)) + ")";
}
void debug_out() {cerr << endl;}
template<typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << ' ' << to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 822
#endif

//head



int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vi lst(n*2+1);
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    lst[a] = b;
    lst[b] = -a;
    if(a > n and b > n) {
      puts("-1");
      return 0;
    }
  }
  int l = 1, r = n*2, ans = 0;
  vi check(n*2+1);
  while(l < r) {
    int cnt = 0, now = 1;
    int u = lst[l];
    if(u < 0) {
      cnt++;
      u = -u;
    }
    check[l++] = check[u] = 1;
    if(r == u) {
      r--;
      continue;
    }
    while(true) {
      vi v;
      for(int i = r; i > u; i--) {
        if(check[i]) continue;
        int r = lst[i];
        if(r < 0) {
          cnt++;
          r = -r;
        }
        check[i] = check[r] = 1;
        v.emplace_back(r);
      }
      int s = v.size();
      now += s;
      r = u-1;
      rep(i, s) {
        if(i and v[i-1] > v[i]) {
          puts("-1");
          return 0;
        }
        if(l == v[i]) l++;
      }
      int y = v.back();
      if(l == y+1) {
        break;
      }
      v.clear();
      for(int i = l; i < y; i++) {
        if(check[i]) continue;
        int r = lst[i];
        if(r < 0) {
          cnt++;
          r = -r;
        }
        check[i] = check[r] = 1;
        v.emplace_back(r);
      }
      s = v.size();
      now += s;
      l = y+1;
      rep(i, s) {
        if(i and v[i-1] < v[i]) {
          puts("-1");
          return 0;
        }
        if(r == v[i]) r--;
      }
      u = v.back();
      debug(l, r, u);
      if(r == u-1) {
        break;
      }
    }
    ans += min(cnt, now-cnt);
  }
  cout << ans << endl;
}