using namespace std;
#include <bits/stdc++.h>
#define BEGIN ios_base::sync_with_stdio(0);cin.tie(0);
#define END return EXIT_SUCCESS;
#define FOR(I,A,B) for(int (I)=(A);(I)<(B);++(I))
#define ALL(C) (C).begin(),(C).end()
int main(int argc,char**argv)
{
	BEGIN
	int N;cin>>N;
	int X,Y,Z;cin>>X>>Y;
	Z=Y-X;if(Z<=0){cout<<-1<<endl;END}
	FOR(i,0,N-1){cin>>X>>Y;if(Z!=Y-X||Z<=0){cout<<-1<<endl;END}}
	cout<<Z<<endl;
	END
}