To merge two sorted list just compare the smallest elements of both list and put smallest element to merged list and remove the selected element from the list and repeat until you get one of the list empty and just copy the another list as it is. This can be done in linear time and worst case will appear when we have to compare the last elements of both list in that case $m+n-1$ comparisions needed.
consider 4 list named 1, 2, 3 & 4 each having $\frac{n}{4}$ elements
Merge 1 and 2 -->comparisions needed = $\frac{n}{4}+ \frac{n}{4} - 1 = \frac{n}{2} - 1$
Merge 3 and 4 -->comparisions needed = $\frac{n}{4}+ \frac{n}{4} - 1 = \frac{n}{2} - 1$
now 1-2 and 3-4 will contain $\frac{n}{2}$ elements each
Merge 1-2 and 3-4 -->comparisions needed = $\frac{n}{2}+ \frac{n}{2} - 1 = n - 1$
$\therefore$, total comparisions needed = $\frac{n}{2} - 1+\frac{n}{2} - 1+n-1 = 2n - 3 $
for n = 400 , comparisions needed = 2(400) - 3 = 797