import bisect
INF = 999
N,Q = map(int, input().split())
S = list(input())
dic = {"A":0,"B":1,"C":2,"D":3}
for i in range(N):
    S[i] = dic[S[i]]

def LIS(L):
    lis = [INF for _ in range(len(L)+1)]
    for l in L:
        lis[bisect.bisect_right(lis,l)] = l
    return len(L)-bisect.bisect_left(lis, INF)

for _ in range(Q):
    query = list(input().split())
    if query[0]=="1":
        x,c = query[1:]
        x = int(x)-1
        S[x] = dic[c]
    else:
        l,r = query[1:]
        l = int(l)
        r = int(r)
        l -= 1
        r -= 1
        print(LIS(S[l:r+1]))