help with python difference between number

[achyuta2017 at gmail.com]

	M <01/14/2019 08:07:01> Count:0 
Total:50
Free: 20
A
B
        M <01/14/2019 08:07:04> Count:1
Total:5
Free:10
A
B
M <01/14/2019 08:07:07> Count:2
Total:5
Free:3
A
B


I am trying to make a output like where it prints the free and then the difference between the current free and previous free
For e.g

M <01/14/2019 08:07:01> Count:0  Free: 20
M <01/14/2019 08:07:04> Count:1  Free: 10  absolute difference between time and prev time is -10
M <01/14/2019 08:07:07> Count:2  Free: 3   absolute difference between time and prev time is -7


And then later on i need to determine the time when we had the most negative free value.


I tried a code like this
Which printed 
 with open("summ4.txt") as f:
# get first line/number
nxt = int(next(f))
for n in f:
    print("absolute difference between {} and {} = {}"
          .format(n.rstrip(), nxt, abs(int(nxt) - int(n))))
    # set nxt equal to the next number
    nxt = int(next(f,0))
   a=open('summ1.txt','r').readlines()
   b=open('summ3.txt','r').readlines()
    with open('summ.txt','w') as out:
      for i in range(0,365): 
print>>out,a[i].rstrip(),b[i]


I hit error as
Traceback (most recent call last):
								File "3.py", line 39, in <module>
							   .format(n.rstrip(), nxt, abs(int(nxt) - int(n))))
								ValueError: zero length field name in format 

I guess my input file has a tab in the start and not able to get a difference rightly.
.
Any pointers on how to achieve the desired result?

Thanks,
Hare