import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        TreeMap<Integer, Integer> map = new TreeMap<>();
        for (int i = 0; i < n; i++) {
            int value = sc.nextInt() + sc.nextInt() * 4;
            map.put(value, map.getOrDefault(value, 0) + 1);
        }
        long max = map.lastKey();
        long ans = 0;
        for (int x : map.keySet()) {
            if (max % 2 != x % 2) {
                System.out.println(-1);
                return;
            }
            ans += (max - x) / 2 * map.get(x);
        }
        System.out.println(ans);
    }
}