数列{an}满足an=n,n=2k-1ak,n=2k,其中k∈N*,设f(n)=a1+a2+a3+...+a2n-1+a2n,则f(2013)-f(2012)等于( ).
Since teenagers have ____ ____these needs, they often question who they are and how they ____ ____ society.