#include<iostream>
using namespace std;
int main(void){
	int len[998], maxlen = 0, n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		int a, b; cin >> a >> b;
		len[i] = a + 4 * b;
		if (len[i] > maxlen) maxlen = len[i];
	}
	long long ans = 0;
	for (int i = 0; i<n; i++){
		if ((maxlen - len[i]) % 2){
			cout << -1 << endl; return 0;
		}
		ans += (maxlen - len[i]) / 2;
	}
	cout << ans << endl;
	system("pause");
	return 0;
}