Why does the following exponent fail in Fortran?

Question

I am using Fortran 77, the g77 compiler.

I have the following.

program title
implicit none
real*4 x,y

y=2
x = (y-4)**.7

end program

However this doesn't seem to be working. If I do: -2^0.7 it works, but not if I do (-2)**.7. Why is that? I only ask because I am writing a program where y is a variable and it will go below 4, so I can't just write -2^0.7

Thanks!