How to insert something to a file from another file with shell?

complex.gro

Great Red Owns Many ACres of Sand 
 6340
    6SER      N    1   2.724  13.826   0.957
    6SER     H1    2   2.685  13.863   1.041
    6SER     H2    3   2.783  13.748   0.979
   .
   .
   .
  405TYR      C 6338   4.079  11.113   0.908
  405TYR    OC1 6339   4.166  11.065   0.896
  405TYR    OC2 6340   4.045  11.204   0.829
   8.10576   5.73277   5.32581

lig.gro

MOL_GMX.gro created by acpype (Rev: 10101) on Tue Nov 14 21:43:44 2017
 37
    1  MOL   C1    1   5.094  10.087   2.868
    1  MOL   C2    2   4.998  10.172   2.807
    1  MOL   C3    3   4.940  10.136   2.690
   .
   .
   .
    1  MOL  H15   35   5.432  10.090   3.227
    1  MOL  H16   36   5.389  10.124   2.993
    1  MOL  H17   37   5.482   9.982   2.948
   15.26200    11.53200    18.72600

I want insert the red part of lig.gro in the complex.gro before the last line
and
change the 2nd line number in complex.gro(6340+37)

the result is like next
complex.gro

Great Red Owns Many ACres of Sand 
 6377
    6SER      N    1   2.724  13.826   0.957
    6SER     H1    2   2.685  13.863   1.041
    6SER     H2    3   2.783  13.748   0.979
   .
   .
   .
  405TYR      C 6338   4.079  11.113   0.908
  405TYR    OC1 6339   4.166  11.065   0.896
  405TYR    OC2 6340   4.045  11.204   0.829
    1  MOL   C1    1   5.094  10.087   2.868
    1  MOL   C2    2   4.998  10.172   2.807
    1  MOL   C3    3   4.940  10.136   2.690
   .
   .
   .
    1  MOL  H15   35   5.432  10.090   3.227
    1  MOL  H16   36   5.389  10.124   2.993
    1  MOL  H17   37   5.482   9.982   2.948
   8.10576   5.73277   5.32581

How can I reach the result by using shell script

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Here is an approach using awk:-

awk '
        NR == FNR {
                if ( FNR == 2 )
                        lig_atoms = $1
                if ( FNR > 2 )
                        atom[++i] = $0
                next
        }
        {
                if ( FNR == 2 )
                {
                        com_atoms = $1
                        $0 = com_atoms + lig_atoms
                }
                if ( FNR > 2 )
                        ++j
                if ( j > com_atoms )
                {
                        for ( c = 1; c < i; c++ )
                                print atom[c]
                }
                print
        }
' lig.gro complex.gro

Try:

awk '
  NR==FNR {
    if(FNR==2)
      ligval=$1
    if(FNR>2 && NF>3)
      lig=lig $0 RS
    next
  }
  {
    if(FNR==2)
      $0=$1+ligval
    if(NF==3)
      $0=lig $0
    print
  }
'  lig.gro complex.gro

Assuming that the last line of a file has 3 fields.

{ head -n -1 complex.gro; cat lig.gro; tail -n 1 complex.gro; } >combined.gro

so tidy!!!