import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] a = new int[N];
		int[] b = new int[N];

		for (int i = 0; i < N; i++) {
			a[i] = sc.nextInt();
			b[i] = sc.nextInt();
		}
		sc.close();

		int max = 0;
		for (int i = 0; i < N; i++) {
			max = Math.max(a[i] + 4 * b[i], max);
		}
		int c = 0;
		for (int i = 0; i < N; i++) {
			if ((max - a[i] - 4 * b[i]) % 2 == 0) {
				c += (max - a[i] - 4 * b[i]) / 2;
			} else {
				System.out.println(-1);
				return;
			}
		}
		System.out.println(c);

	}
}