import java.util.Scanner;

public class No178 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int max = 0, N, ans = 0;
        N = Integer.parseInt(sc.next());
        int[] a = new int[N];
        int[] b = new int[N];

        for (int i = 0; i < N; i++) {
            a[i] = Integer.parseInt(sc.next());
            b[i] = Integer.parseInt(sc.next());
            max = Math.max(max, a[i] + 4 * b[i]);
        }

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

        System.out.println(ans);
    }
}