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

int main(void){
	int n;
	cin >> n;
	int ret = 0;
	for(int i=0;i<n;i++){
		int x,y;cin >> x >> y;
		if(i == 0)ret = y - x;
		else{
			if(ret == y-x)ret = y - x;
			else ret = -1;
		}
	}
	cout << (ret<=0?-1:ret) << endl;
	return 0;
}