Longest Substring with a and b count

Longest Substring with a and b count: A string S containing only the characters a and b is passed as the input. The program must print the length L of the longest substring with equal numbers of a and b in it.

Note: Optimize the algorithm so that the program executes successfully within the time limit.

Boundary Condition(s):
1 <= Length of string S<= 100000

Input Format:
The first line contains the string S.

Output Format:
The first line contains the L.

Example Input/Output 1:
Input:
abaabba

Output:
6

Example Input/Output 2:
Input:
aaabaaabbbaaab

Output:
8

def stringLen(str):
m = dict()
m[0] = -1
ca = 0
cb = 0
res = 0
for ele in range(len(str)):
if str[ele] == 'a':
ca += 1
else:
cb += 1
if m.get(cb - ca):
res = max(res, ele - m[cb - ca])
else:
m[cb - ca] = ele
return res
stri = input()
print(stringLen(stri))