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());
    
    Long[] a = new Long[N]; 
    Long[] b = new Long[N]; 
    Long[] sum = new Long[N+1]; 

    while(true){
        a[cnt] = Long.parseLong(sc.next());
        b[cnt] = Long.parseLong(sc.next());
        cnt++;
        if(cnt==N)break;
    }
    
    sum[0] = (long)0;
    for( int i = 0; i < N ; i++ ){
      
        sum[i+1] = a[i] + ( 4 * b[i] );

    }
    
    Arrays.sort( sum );
    Long max = sum[N];
    long d = 0;
    
    for( int i = 1; i < N ; i++ ){
         d += ( max - sum[i] ) / 2 ;
    }
    
    if(  d % 2 == 1 && max % 2 == 0  ) System.out.print( -1 );
    else if(  max % 2 == 1 && d % 2 == 0  ) System.out.print( -1 );
    else         System.out.print(  d );
        System.out.println();
        
	}
}