Python 3 - My code gets timed out for higher conditions can anyone help me optimise it

Question

#!/bin/python3

import math
import os
import random
import re
import sys
mat = []
a = 0
c =0

# Complete the cipher function below.
def cipher(n,k, s):
global a
global c
global mat
mat.append(s[0])
for i in range(1,n):

if i < k:
a = 0
for j in range (i):
a += int(mat[j],2)
e = bin(a)
b = e[-1]

if s[i] == b:
mat.append("0")
else:
mat.append("1")
else:
c = 0
q = i-k+1
for q in range(i-k+1,i):
c += (int(mat[q],2))
f = bin(c)
d = f[-1]
if s[i]== d:
mat.append("0")
else:
mat.append("1")
return "".join(mat)

if __name__ == '__main__':
fptr = open(os.environ['OUTPUT_PATH'], 'w')

nk = input().split()

n = int(nk[0])

k = int(nk[1])

s = input()

result = cipher(n,k, s)

fptr.write(result + '\n')

fptr.close()

Answer 1

Is this a homework assignment?

At the very least you should create a CodePlayground code so that we can test it.
When you do this you may have to fix some whitespace errors (sometimes these are introduced by copy+pasting).
~ Just delete any syntax errors pointing to spaces--they're HTML spaces, not code spaces--and then put the spaces back normally.

Note: The os.environ variable you're querying does not exist on SoloLearn.

Also, what inputs does it take?
The inputs I tried as guesses just made it crash, not time out.