#include <algorithm>
#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <map>
#define rep(i, n) for (int i=0; i<n; ++i)
#define FOR(i, a, b) for (int i=a; i<b; ++i)
#define pb(x) push_back(x)
#define ll long long
#define ul unsigned long long
#define debug(x) cout  << #x << " : " << x << endl;
using namespace std;

const int Inf =1500000000;
int main () {
	int n;
	cin >> n;
	int diff;
	bool flg;
	rep(i, n) {
	  int x, y;
	  cin >> x >> y;
	  flg = false;
	  if (i == 0) diff = y - x > 0 ? y - x : -1;
	  else if (diff != y - x) break;
	  flg = true;
	}
	cout << (flg ? diff : -1) << endl;
	return 0;
}