n, k = map(int, input().split())
s = input()[:-1]
count = 0
if k < 0:
    count -= k
    k = 0
for i in s:
    if i == 'A':
        k += 1
    elif i == 'C':
        k -= 1
        if k < 0:
            count += 1
            k += 1
print(count)