#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin();i!=(c).end();i++)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double EPS = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};

int N;

int main() {
	// cin.tie(0); // tie
	// ios::sync_with_stdio(false); // sync
	cin >> N;
	int a[N], b[N];
	int maxi = -1;
	rep(i, N) {
		cin >> a[i] >> b[i];
		maxi = max(maxi, a[i] + 4 * b[i]);
	}

	int ans = 0;
	rep(i, N) {
		if ((maxi - (a[i] + 4 * b[i])) % 2 != 0) {
			ans = -1;
			break;
		}
		ans += (maxi - (a[i] + 4 * b[i])) / 2;
	}

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