I=input
S=I()
S=I()
A=C=a=n=0
L='A'
for s in S:
	if s==L:A+=1
	else:C,n=C+1,n+A
c=C
m=n
for s in S:
	if s<L:n,a,c=n+c-1-a,a+1,c-1
	c-=s>L
	a+=s==L
	m=max(n,m)
print(m)