#include <algorithm>
#include <iostream>
#include <iomanip>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <cmath>
using namespace std;

#define FOR(i,s,e) for (int i = int(s); i != int(e); i++)
#define FORIT(i,c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define ISEQ(c) (c).begin(), (c).end()



int main(){
	int N;
	cin >> N;
	int X[N],Y[N];
	FOR(i,0,N) cin >> X[i] >> Y[i];

	int ans = Y[0] - X[0];
	if (ans <= 0) ans = -1;
	FOR(i,1,N){
		if(ans <= 0) break;
		else{
			if(ans != Y[i] - X[i]) ans = -1;
		}
	}
	cout << ans << endl;
	return 0;
}