#include<iostream>
#include<string>
#include<algorithm>
#include<functional>
#include<vector>
#include<cstdio>
#include<cstring>
#include<cstring>
#include<cmath>
#include<stack>
#include<queue>
#include<set>
#include<map>
#define int long long
using namespace std;

int n;
int a, b;
int x[1000];

signed main() {
	int i;
	cin >> n;
	for( i = 0; i < n; i++ ) {
		cin >> a >> b;
		x[i] = a + 4*b;
	}
	sort(x, x+n, greater<int>() );
	
	int mx = x[0], ans = 0;
	for( i = 1; i < n; i++ ) {
		if ( (mx - x[i])%2 ) {
			cout << -1 << endl;
			return 0;
		}
		ans += (mx - x[i])/2;
	}
	cout << ans << endl;
	return 0;
}