import java.util.*;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        int [] a = new int[N];
        int [] b = new int[N];
        int [] c = new int[N];
        int ans = 0;
        int max = 0;
        int num = -1;
        boolean x = false, y = false;

        for (int i = 0; i < N; i++) {
            a[i] = scan.nextInt();
            b[i] = scan.nextInt();
            
            if (a[i]%2==0) x = true;
            else y = true;
            if (x && y) break;
            
            c[i] = a[i] + 4*b[i];
            if (max < c[i]) {
                num = i;
                max = c[i];
            }
        }

        for (int i = 0; i < N; i++) {
            if (x && y) {
                ans = -1;
                break;
            }
            ans += (c[num] - c[i])/2;
        }
        
        System.out.println(ans);
    }
}