#include <algorithm>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
typedef long long ll;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)

int main(){
  int n;
  cin >> n;
  bool ok = true;
  int x, y;
  cin >> x >> y;
  int sa1 = y - x;
  FOR(i,0,n-1){
    cin >> x >> y;
    int sa2 = y - x;
    if(sa1!=sa2) ok = false;
  }
  cout << (ok ? sa1 : -1) << endl;
  return 0;
}