#include <iostream>
#include <iomanip>

#include <algorithm>
#include <math.h>

#include <string>
#include <list>
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

int main(void){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed;//
    //cout << setprecision(6);//

    int N;
	cin>>N;//[1,10^3]
	int x[1000],y[1000];
	int ans = -1;
	int temp;
	rep(i,N){
		cin>>x[i]>>y[i];
		if(x[i]>y[i]){P(-1);return 0;}
		temp = y[i]-x[i];
		if( ans != -1 && ans != temp){
			P(-1);return 0;
		}
		ans = temp;
	}
	if(ans == 0){
		P(-1);
	}else{
		P(ans);
	}
    return 0;
}