#include<iostream>
using namespace std;
int main(void)
{	
	long long int i,n, ans=0, max=-1;
	long int space[1100][2],kk[1100];

	cin >> n;
	for (i = 0; i < n; i++) {
		cin >> space[i][0] >> space[i][1];
		kk[i] = space[i][0] + 4 * space[i][1];
		if (max < kk[i])max = kk[i];
	}
	
	for (i = 0; i < n; i++) {
		ans = ans + (max - kk[i]);
		
		if ((max - kk[i]) % 2 != 0)
		{
			ans = -2;
			break;
		}
		
	}
	cout << ans / 2;

	return 0;
}