The variable command

Dear LAMMPS users and developers:
    I am using lammps to do my project.I found a problem in my input script.The following is part of my input script.
#for loop
label forloop
variable i loop 20
variable f equal v_i*0.00025558
fix 7 mobile addforce 0.0 0.0 v_f
run 20000
next i
jump in.graphene forloop
run 40000
When the loop was over,an error(Invalid variable name in variable formula (variable.cpp:1098))came to the screen and the command-run 40000 couldn't continue to run.I know there's something wrong with the variable command.But i don't know how to solve it.Can you help me to find the problem?
    Thank You!