ssh tunnel with 2 hops and key

Applications IP Networking
1

I have situation:

  • localhost L
  • server A
  • server B (currently accessible only from B, but it have key of my localhost - added when L have different location and can access directly to B)

problem:
how set tunnel from L to B and use key from localhost (I don't remember password to B)?

I have tried:
ssh -f -N -L localPORT:serverB:22 -2 mylogintoA@serverA
but it doesn't work for me.