from atcoder import segtree
N,Q=map(int,input().split())
S=input()
S=list(S)
"""
a
b
c
d
ab
bc
cd
da
abc
bcd
d
"""
change = []
for i in range(4):
    for j in range(4):
        if j + i < 4:
            change.append((j,j+i))
move = []
for k in range(len(change)):
    for t in range(len(change)):
        if change[k][1] <= change[t][0]:
            new = (change[k][0],change[t][1])
            move.append((k,t,change.index(new)))

s = "ABCD"
def one(n):
    count = 0
    res = []
    for i in range(4):
        for j in range(4):
            if j + i < 4:
                count += 1
                if j <= n <= j+i:
                    res.append(0)
                else:
                    res.append(1)
    return tuple(res)


def op(a,b):
    new = [10**10 for _ in range(10)]
    for a_,b_,c in move:
        new[c] =min(new[c], a[a_]+b[b_])
    return tuple(new)

st = segtree.SegTree(op=lambda a,b:op(a,b),e=tuple(0 for _ in range(10)),v=[one(s.index(S[i])) for i in range(N)])
for _ in range(Q):
    q=list(map(str,input().split()))
    if q[0] == "1":
        _,x,c = q
        x=int(x)
        st.set(x-1,one(s.index(c)))
    else:
        _,l,r = q
        l=int(l)
        r=int(r)
        ans=st.prod(l-1,r)
        print(min(ans))