#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

int digisum(int i){
    int sum1 = 0;
    while (i) {
        sum1 += i%10;
        i /= 10;
    }
    return sum1;
}

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