#include <bits/stdc++.h>

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORTASC(c) sort((c).begin(),(c).end())
#define SORTDESC(c,t) sort((c).begin(),(c).end(), std::greater<t>());
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

const double PI  = acos(-1.0);

using ll = long long;
using namespace std;

int main() {
    ll n;
    vector<ll> v;
    cin >> n;

    ll a, b;
    for (ll i = 0; i < n; i++){
        cin >> a >> b;
        v.push_back(a + b*4);
    }

    SORTDESC(v, ll);
    
    ll ans = 0;
    for (ll i = 1; i < n; i++){
        if((v[0]-v[i]) % 2 == 0){
            ans += (v[0]-v[i]) / 2;
        } else {
            ans = -1;
            break;
        }
    }

    cout << setprecision(20) << ans << endl;

    return 0;
}