#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main()
{
	vector<long long>p;
	int n;
	cin >> n;
	for (int i = 0; i < n; i++){
		int x, y;
		cin >> x >> y;
		if (n == 1&&y-x>=0){
			cout << y - x << endl;
			return 0;

		}
		if (y - x > 0){
			p.push_back(y - x);
		}
	}
	sort(p.begin(), p.end());
	bool flag = false;
	for (int i = 0; i < p.size()-1&&p.size()>1; i++){
		if (p[i] == p[i + 1]){
			int c=0;
			for (int i2 = i+1; i2 < i + n; i2++){
				if (p[i] == p[i2]){
					c++;
				}
			}
			if (c == n - 1){
				cout << p[i] << endl;
				flag = true;
			}
		}
	}
	if (!flag){
		cout << "-1" << endl;
	}
}