#include <iostream>
#include <sstream>
#include <math.h>
#include <stack>
#include <set>
#include <algorithm>
#include <string>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <vector>
using namespace std;

#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	int n,x,y;
	cin >> n;
	int ans;
	for(int i = 0;i < n;i++){
		cin >> x >> y;
		if(i == 0) ans = y-x;
		if(y-x != ans) {
			ans = INF;
			break;
		}
	}
	if(ans < 0) ans = INF;
	if(ans == INF) cout << "-1" << endl;
	else cout << ans << endl;
	return 0;
}