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

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll N;
    cin >> N;

    vector<ll> A(N);
    vector<ll> B(N);
    set<ll> se;
    FOR(i, 0, N){
        ll a; cin >> a;
        A[i] = a;
        se.insert(a%2);
        cin >> B[i];
    }
    if(se.size()==2){
        p(-1);
        return 0;
    }

    ll ma = 0;
    FOR(i, 0, N){
        ll c = A[i] + 4*B[i];
        chmax(ma, c);
    }

    ll count = 0;
    FOR(i, 0, N){
        ll c = A[i] + 4*B[i];
        ll diff = ma - c;
        count += diff/2;
    }

    p(count);
    
    return 0;
}