#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
#define allpt(v) (v).begin(), (v).end()
#define allpt_r(v) (v).rbegin(), (v).rend()

const int mod = 1e9 + 7;
const string wsp = " ";
const string tb = "\t";
const string rt = "\n";

template <typename T>
void show1dvec(vector<T> v)
{
    if (v.size() == 0)
        return;
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << wsp;
    cout << v[n] << rt;
    return;
}

template <typename T>
void show2dvec(vector<vector<T>> v)
{
    int n = v.size();
    rep(i, n) show1dvec(v[i]);
}

bool in_field(int h, int w, int y, int x)
{
    return 0 <= y && y < h && 0 <= x && x < w;
}

ll pow_mod(int p, int q, int mod)
{
    ll ret, r;
    ret = 1;
    r = p;

    while (q > 0)
    {

        if (q % 2)
        {
            ret *= r;
            ret %= mod;
        }

        r = (r * r) % mod;
        q /= 2;
    }

    return ret % mod;
}

int main()
{

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

    int n, s, t, maxwidth;
    cin >> n;
    ll ans = 0;
    bool ok = true;
    VI width;
    rep(i, n)
    {
        cin >> s >> t;
        width.push_back(s + 4 * t);
    }
    maxwidth = *max_element(allpt(width));
    rep(i, n)
    {
        ans += (maxwidth - width[i]) / 2;
        if ((maxwidth - width[i]) % 2) ok = false;
    }
    cout << (ok ? ans : -1) << rt;

    return 0;
}