#include <bits/stdc++.h>
using namespace std;
#define REP(i,N) for(int i=1;i<N;i++)
int main() {
	int N,sub;
	cin>>N;
	int X[N],Y[N];
	cin>>X[0]>>Y[0];
	sub=Y[0]-X[0];
	if(X[0]>=Y[0]){
		sub=-1;
	}
	REP(i,N){
		cin>>X[i]>>Y[i];
		if(sub==Y[i]-X[i] && X[i]<Y[i]){
			continue;
		}else{
			sub=-1;
			break;
		}
	}
	cout<<sub<<endl;
	return 0;
}