#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <climits>
#include <vector>
#include <numeric>
#include <complex>
using namespace std;

//#define __int64 long long
#define long __int64
#define REP(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
const int Vecy[4] = {0,-1,0,1};
const int Vecx[4] = {1,0,-1,0};

int main(){
	int n;
	int x[1000],y[1000];
	int ans;
	bool flg = true;
	cin >> n;
	
	for(int i=0; i<n; i++){
		cin >> x[i] >> y[i];
	}
	
	ans = y[0] - x[0];
	
	if(ans <= 0){
		flg = false;
	}
	
	for(int i=1; i<n; i++){
		if(ans != y[i] - x[i]){
			flg = false;
			break;
		}
	}
    
	if(!flg){
		cout << -1 << endl;
	}else{
		cout << ans << endl;
	}
	
    return 0;
}