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

int main(void) {
	int n;
	cin >> n;
	vector<int> a(n),b(n);
	int l = 0;
	for(int i=0;i<n;i++){
		cin >> a[i] >> b[i];
		l = max(a[i]+b[i]*4,l);
	}

	int sum = 0;
	for(int i=0;i<n;i++){
		if( (l -  a[i]-b[i]*4) % 2 != 0){
			cout << -1 << endl;
			return 0;
		}
		sum += (l - a[i] - b[i] * 4)/2;
	}
	cout << sum << endl;

	return 0;
}