import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner (System.in);
        int n = koko.nextInt();
        int[] a = new int[n];
        int[] b = new int[n];
        for(int i =0; i<n; i++){
            a[i] = koko.nextInt();
            b[i] = koko.nextInt();
        }
        int max=0;
        int[] white = new int[n];
        for(int i =0; i<n; i++){
            white[i] = a[i]+4*b[i];
            max = Math.max(max,white[i]);
        }
        long need=0;
        for(int i=0; i<n; i++){
            if((max-white[i])%2==0){
            need = need + (max-white[i])/2;
            }else{
                need = -1;
                break;
            }
        }
        System.out.println(need);

    }
}