#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 2000000000

int main(){	
	
	int n;
	cin>>n;
	
	vector<long long> a(n),b(n);
	long long m = 0;
	rep(i,n){
		cin>>a[i]>>b[i];
		m = max(m,a[i]+b[i]*4);
	}
	long long ans = 0;
	rep(i,n){
		if(m%2!=(a[i]+b[i]*4)%2){
			cout<<-1<<endl;
			return 0;
		}
		ans += m - (a[i]+b[i]*4);
	}
	cout<<ans/2<<endl;
	
	return 0;
	
}