#include"bits/stdc++.h"
using namespace std;


int main() {
	int N;
	cin >> N;
	vector<int> X(N),Y(N);
	for (int i = 0; i < N; i++) cin >> X[i] >> Y[i];

	bool flag=true;
	int sq;
	for (int i = 0; i < N; i++) {
		if(i==0) sq = Y[i]-X[i];
		if (sq <= 0) {
			flag=false;
			break;
		}
		else {
			if (sq != (Y[i] - X[i])) {
				flag=false;
				break;
			}
		}
	}

	cout << (flag?sq:-1) << endl;

}