#include<map>
#include<set>
#include<list>
#include<cmath>
#include<queue>
#include<stack>
#include<cstdio>
#include<string>
#include<vector>
#include<complex>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<numeric>
#include<sstream>
#include<iostream>
#include<algorithm>
#include<functional>
#include<cctype>

#define mp make_pair
#define pb push_back
#define REP(i,a,n) for(int i = a;i < (n);i++)
#define rep(i,n) for(int i = 0;i < (n);i++)
#define all(s) s.begin(), s.end()
#define rall(s) s.rbegin(), s.rend()
#define range(x,min,max) ((min) <= (x) && (x) <= (max)) 

using namespace std;

typedef long long LL;
typedef vector<int> VI;
typedef vector<vector<int> > VII;
typedef pair<int,int> PII;

const int DX[]={1,0,-1,0},DY[]={0,-1,0,1};

int main(){
	cin.tie(0);
   	ios::sync_with_stdio(false);
	int N;
	int ans = 0;
	VI xs, ys;
	cin >> N;
	rep(i, N){
		int x, y;
		cin >> x >> y;
		xs.pb(x);
		ys.pb(y);
	}
	rep(i, N){
		if(ys[i] - xs[i] < 0 || (ans != 0 && ans != ys[i] - xs[i])){
			ans = -1;
			break;
		} else {
			ans = ys[i] - xs[i];
		}
	}
	cout << ans << endl;
	return 0;
}