//
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P>IP;
typedef vector<ll> V;
typedef vector<V> V2;
typedef vector<vector<P> > G;
void g_dir(G &graph, ll a, ll b, ll w = 1){graph[a].push_back(P(b, w));}
void g_undir(G &graph, ll a, ll b, ll w = 1){g_dir(graph, a, b, w);g_dir(graph, b, a, w);}
#define rep(i, n) for(ll (i) = 0; (i) < (n); (i)++)
#define rep1(i, n) for(ll (i) = 1; (i) <= (n); (i)++)
#define rrep(i, n) for(ll (i) = (n) - 1; (i) >= 0; (i)--)
#define rrep1(i, n) for(ll (i) = (n); (i) >= 1; (i)--)
template<class T> void chmax(T &a, const T &b){if(a < b){a = b;}}
template<class T> void chmin(T &a, const T &b){if(a > b){a = b;}}
const ll INF = 1145141919;
const ll MOD = 1000000007;
const ll NUM = 101010;

bool is_uruu(int y) {
    return (y % 4 == 0 && y % 100 != 0 || y % 400 == 0) ? true : false;
}

int cal[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};

int lim(int y, int m){
    if(is_uruu(y) && m == 2)return 29;
    return cal[m];
}

int main(){

    ll N;
    cin >> N;
    V v(N);
    rep(i, N){
        ll a, b;
        cin >> a >> b;
        v[i] = a + b * 4;
    }
    sort(v.begin(), v.end(), greater<ll>());
    ll ans = 0;
    rep(i, N){
        if((v[0] - v[i]) % 2){
            cout << -1 << endl;
            return 0;
        }
        ans += v[0] - v[i];
    }
    cout << ans / 2 << endl;
    return 0;
}