#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define SZ(n) (int)(n).size()
using namespace std;
typedef long long ll;
typedef vector<int> VI;

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