n, mood = map(int, input().split())
ans = 0; s = list(input().replace('B', ''))
if mood < 0:
	ans += -mood
	mood = 0
for i in s:
	if i == 'A':
		mood += 1
	else:
		if mood > 0:
			mood -= 1
		else:
			ans += 1
print(ans)