I kinda stole $$ wut do i do know?

ok so i had 2 of my frends over and as a joke i took $20 frum girl(1) purses and i told the other girl(2) that i took it (i was planning on giving it back to girl1 asap becuz she didnt notice me take it) but then girl2 told me (dared me i ges) to keep it til the end of the night to c if she noticed. being the dummy i am i took the 'dare'. well girl1 never noticed but at the end of the nite wen i was gna giv it back i could not find it! uh oh! i havent told girl1 yet becuz i dont know what to do. should i get another $20 (which isnt easy for a 13 year old i dont get an allowance :( (me) and girl1 is 18 (i know shes way older than me but weve ben frends for like ever! hahaha).)? or should i just let her keep being totally oblivious of the fact that i took the money? i know that the right thing to do is the pay her back but i dont wna (know one wants to work lol!!) anyways honestly wut wud u do?