Da

import math
n, k = map(int, input().split())
s = input()
count = 0
if k < 0:
    count += int(math.fabs(k))
    k = 0

for i in s:
    if k == -1:
        count += 1
        k += 1
    if i == 'A':
        k += 1
    elif i == 'B':
        continue
    else:
        k -= 1
print(count)