my code :
n = input()
num = [0 for i in xrange(n) ]
for i in xrange(n):
    x, y = map(int, raw_input().split())
    num[i] = x+y

cnt = 0
for i in xrange(n-1, -1, -1):
    num[i] += cnt
    cnt = 0
    if num[i] > 9  and i > 0:
        num[i] -= 10; cnt = 1
print ''.join( map(str, num) )

As explained in another post, for the following set of data:

4
9 0
5 4
5 4
5 5

the program has to return:
0000

Your code is wrong since it returns:
10000

The answer to

4
9 0
5 4
5 4
5 5

is 10000.