#include <bits/stdc++.h>
#define LL long long
#define ULL unsigned long long
#define REP(i,n) for(int i=0; i<(n); i++)
#define REP2(i,x,n) for(int i=x; i<(n); i++)
#define SORT(n) sort((n).begin(), (n).end(), greater<int>())
using namespace std;
int main (){
     
    cin.tie(0);
    ios::sync_with_stdio(false);
	
	int N;
	cin >> N;

	vector<LL> A(N), B(N), sum(N);
	LL ma{};
	REP( i, N ){
		cin >> A[i] >> B[i];
		sum[i] = A[i] + B[i] * 4;
		ma = max( ma, sum[i] );
	}
	LL res{};
	bool flag = false;
	REP( i, N ){
		if( ( ma - sum[i] ) % 2 == 0 ){
			res += ( ma - sum[i] ) / 2;
			flag = true;
		}else{
			flag = false;
			break;
		}
	}
	cout << ( flag ? res : -1 ) << endl;
	return 0;
}