how careless....

my exams for the semester are going on..
before my third paper begins, while writing my exam index number i just glance at my exam docket to check if i am writing it correctly. my sense was right, i had written my last two digits wrongly...it should be 31 but i wrote 13.. . then i thought to myself that i have most probably written that number wrongly for my other two papers before this.damn!!!

how could i be so careless????