#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    ll n;
    cin >> n;
    ll a[n], b[n];
    ll x[n];
    rep(i,n){
        cin >> a[i] >> b[i];
        x[i] = a[i] + 4 * b[i];
    }
    sort(x, x+n);
    ll ans = 0;

    rep(i,n){
        if((x[n-1] - x[i]) % 2 == 1){
            ans = -1;
            break;
        }
        ans += (x[n-1] - x[i]) / 2;
    }

    cout << ans << endl;
    return 0;
}