/* -*- coding: utf-8 -*-
 *
 * 185.cc: No.185 和風 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

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

  int x0, y0;
  cin >> x0 >> y0;

  int z = y0 - x0;
  if (z <= 0) {
    puts("-1");
    return 0;
  }

  for (int i = 1; i < n; i++) {
    int xi, yi;
    cin >> xi >> yi;
    if (z + xi != yi) {
      puts("-1");
      return 0;
    }
  }

  printf("%d\n", z);
  return 0;
}