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

int main()
{
	int N;
	cin >> N;
	vector<long long> a( N );
	vector<long long> b( N );
	long long ma = 0;
	for( int i = 0; i < N; i++ ) {
		cin >> a[i] >> b[i];
		long long s = a[i] + b[i] * 4;
		ma = max( ma, s );
	}

	long long ans = 0;
	for( int i = 0; i < N; i++ ) {
		long long s = a[i] + b[i] * 4;
		long long n = ma - s;
		if( n % 2 ) {
			ans = -1;
			break;
		}
		ans += n / 2;
	}

	cout << ans << endl;
}