python, first job application I have written to achieve the calculation of the text file and save it as another file format

Question: an existing text file evt_num.txt, document reads as follows:

1999-7-15    1  
1999-8-15    1   
1999-9-15    5   
1999-10-15    4 
1999-11-15    3 
1999-12-15    0 

Demand: increases file a third column contents of the accumulated value of the second column contents, i.e. c3 [0] = 1, c3 [1] = 2, c3 [2] = 7, c3 [3] = 9 .. ....

Specific codes are as follows:

f1 = open('evt_num.txt','r')
f2 = open('new_evt_num.txt','w')
c1 = []
c2 = []
C3 = []
count = 0
s = 0
for i in f1:
    x,y,z = i.strip().split()
    c1.append(x)
    c2.append(y)
    s += int(y)
    c3.append(s)
    line = str(c1[count])+'\t'+str(c2[count])+'\t'+str(c3[count])+'\n'
    f2.write(line)
    count += 1
f1.close()
f2.close()

After executing the code to generate a new file new_evt_num.txt, as follows:

1999-7-15    1    1
1999-8-15    1    2
1999-9-15    5    7
1999-10-15    4    11
1999-11-15    3    14
1999-12-15    0    14

This code is easy to understand, the main application file read and write operations, string segmentation, list operations, string concatenation like.