#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;


int main(){
  int N;
  cin >> N;
  map<int,int> memo;
  rep(i,N){
    int X, Y;
    cin >> X >> Y;
    if(X>=Y){
      cout << -1 << endl;
      return 0;
    }
    memo[Y-X]++;
  }

  if(memo.size() != 1){
    cout << -1 << endl;
  }else{
    FOR(itr,memo){
      cout << itr->first << endl;
    }
  }
  return 0;
}