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

#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define rev(i,n) for(int (i)=(n-1);(i)>=0;(i)--)
#define repp(i,n) for(int (i)=1;(i)<=(n);(i)++)
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rev(i,n) for(int (i)=(n-1);(i)>=0;(i)--)
#define clr(a) memset((a), 0 ,sizeof(a))


typedef pair<int,int> P;
typedef vector<pair<int,string> > pii;
typedef map<string,int> mi;

int main() {
  int n,ans,x,y;
  cin >> n;
  cin >> x >> y;
  ans = y-x;
  if (ans <= 0) {
    cout << -1 << endl;
    return 0;
  }
  for(int i=0;i<n-1;i++){
  	cin >> x >> y;
  	if (ans!=y-x){
  		cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
  return 0;
}