n = int(input())
db = int(n/2)
lista = list(input())
sega = [0]*n
segb = [0]*n
if lista[0] == "A":
    sega[0] = 1
    segb[0] = 0
else:
    segb[0] = 1
    sega[0] = 0
maxa = 0
maxb = 0
for i in range(1, n):
    if lista[i]=="A":
        sega[i] = sega[i-1] +1
        segb[i] = segb[i-1]
    else:
        sega[i] = sega[i-1] 
        segb[i] = segb[i-1] +1
    if min(maxa, maxb) < min(sega[i], db-segb[i]):
        maxa = sega[i]
        maxb = db - segb[i]
print(min(maxa, maxb)*2)