#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <set>
#include <iomanip>
using namespace std;
using ll = long long;

int main() {
    int N;
    cin >> N;
    set<int> s;
    for(int i=0; i<N; ++i) {
        int x, y;
        cin >> x >> y;
        s.insert(y-x);
    }
    if(s.size() != 1 || *s.begin() <= 0) {
        cout << -1 << endl;
    } else {
        cout << *s.begin() << endl;
    }
}