N = int(input())
S = input()

cum = [0]
cum2 = [0]
IDX = []
SUM = 0
for i, s in enumerate(S):
    cum.append(cum[-1])
    cum2.append(cum2[-1])
    if s == "A":
        cum[-1] += 1
    if s == "C":
        cum2[-1] += 1
        SUM += cum[-1]
    if s == "?":
        IDX.append(i)
        SUM += cum[-1]

ans = SUM
for i, idx in enumerate(IDX):
    SUM += -cum[idx]+(cum2[-1]-cum2[idx+1])
    ans = max(ans, SUM+(i+1)*(len(IDX)-(i+1)))

print(ans)