#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <set>
#include <sstream>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
using namespace std;

typedef long long ll;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)
#define foreach(itr,c) for(__typeof(c.begin()) itr=c.begin(); itr!=c.end(); itr++)

int main(int argc, char const *argv[]) {
  int n;
  cin >>n;

  int ans=0;

  REP(i,n){
    int x,y;
    cin >>x >>y;

    if(ans>=0){
      int t=y-x;

      if(t>0){
        if(ans>0){
          if(ans!=t) ans=-1;
        }
        else ans=t;
      }
      else ans=-1;
    }

  }

  std::cout << ans << std::endl;
  return 0;
}