import java.util.*;

public class Exercise103{
  public static void main (String[] args){

    Scanner sc = new Scanner(System.in);

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