#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

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