import java.util.*;

class No0178{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int sp, tb, temp;
		int cnt = 0;
		ArrayList<Integer> a = new ArrayList<Integer>();

		for(int i = 0; i < n; i++){
			sp = sc.nextInt();
			tb = sc.nextInt() * 4;
			a.add(sp + tb);
		}
		
		Collections.sort(a);
		Collections.reverse(a);

		for(int i = 1; i < n; i++){
			temp = a.get(0) - a.get(i);
			if(temp % 2 != 0){
				cnt = -1;
				break;
			}else{
				cnt = cnt + (temp / 2);
			}
		}
		System.out.println(cnt);
	}
}