import java.util.*;
 
public class Main {
	public static void main(String[] args) {
 
	Scanner sc  = new Scanner(System.in);
        
    int cnt = 0;
    boolean huhu = false ;
    
    int N = Integer.parseInt(sc.next());
    
    int[] a = new int[N]; 
    int[] b = new int[N]; 
    int[] sum = new int[N+1]; 

    while(true){
        a[cnt] = Integer.parseInt(sc.next());
        b[cnt] = Integer.parseInt(sc.next());
        cnt++;
        if(cnt==N)break;
    }
    
    for( int i = 0; i < N ; i++ ){
        sum[i] = a[i] +  ( 4 * b[i] );
    }
    
    Arrays.sort(sum);
    int max = sum[N];
    int d = 0;
    
    for( int i = 1; i < N ; i++ ){
         d += ( max - sum[i] ) / 2 ;
    }
    
    if( ( max - sum[1] ) % 2 == 1  ) System.out.print( -1 );
    else         System.out.print(  d );
        System.out.println();
	}
        
}