怎样用fortran求两个数的最大公因数

program main
!辗转相除法
integer m,n
common m,n
print*,'Please input two positive integers:'
read(*,*) m,n
call Euclid(m,n)
endprogram
subroutine Euclid(m,n)
integer r
do while(n.ne.0)
 r = mod(m,n)
 m = n
 n = r
enddo
print*,'Their greatest common divisor is', m
endsubroutine