There are 2n natural numbers 1,2,...,2n. 
Pick up any n natural numbers out of those
 2n natural numbers and order them like:
 
  a1<a2<........<an  ,
 
and order the remaining n integers like:
 
  b1>b2>........>bn  .
 
  Then prove that
 
 |a1-b1|+|a2-b2|+...+|an-bn|=n2 .