[MPICH] how to run only nodes?

[rytis]

Hello,
I am new MPICH2 user, and maybe you can help me.
I am runing Mpich2 on Cluster (16 nodes + Master, each have 2 
processors). When i run mpiexec -n 16 a.out it execute on all nodes 
(including master). i tryied add -1 option, but  it left to run 1 
processor on master. How to run  process, which would work only on  
nodes(not on master)?
Thanks

p.s. sry for my bad english..