class SegTree:
    def __init__(self, op, e, lst):
        self.n = len(lst)
        self.N0 = 2 ** (self.n - 1).bit_length()
        self.op = op
        self.e = e
        self.data = [e] * (2 * self.N0)
        
        for i in range(self.n):
            self.data[self.N0 + i] = lst[i]
        for i in range(self.N0 - 1, 0, -1):
            self.data[i] = self.op(self.data[2*i], self.data[2*i+1])
    
    def set(self, i, x):
        i += self.N0
        self.data[i] = x
        while i > 1:
            i >>= 1
            self.data[i] = self.op(self.data[2*i], self.data[2*i+1])
    
    def prod(self, l, r):
        if r <= l:
            return self.e
        lres = self.e
        rres = self.e
        l += self.N0
        r += self.N0
        while l < r:
            if l & 1:
                lres = self.op(lres, self.data[l])
                l += 1
            if r & 1:
                r -= 1
                rres = self.op(self.data[r], rres)
            l >>= 1
            r >>= 1
        return self.op(lres, rres)

import sys
from collections import defaultdict
input = sys.stdin.readline

inf = 10**6
def op(l, r):
	return (l[0] + r[0], min(l[0] + r[1], l[1] + r[4]), min(l[0] + r[2], l[1] + r[5], l[2] + r[7]), min(l[0] + r[3], l[1] + r[6], l[2] + r[8], l[3] + r[9]), l[4] + r[4], min(l[4] + r[5], l[5] + r[7]), min(l[4] + r[6], l[5] + r[8], l[6] + r[9]), l[7] + r[7], min(l[7] + r[8], l[8] + r[9]), l[9] + r[9])

e = (0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

d = defaultdict(tuple)
d['A'] = (0, 0, 0, 0, 1, 1, 1, 1, 1, 1)
d['B'] = (1, 0, 0, 0, 0, 0, 0, 1, 1, 1)
d['C'] = (1, 1, 0, 0, 1, 0, 0, 0, 0, 1)
d['D'] = (1, 1, 1, 0, 1, 1, 0, 1, 0, 0)

N, Q = map(int, input().split())
S = input()[:N]
v = [d[i] for i in S]
seg = SegTree(op, e, v)
for _ in range(Q):
	t, a, b = input().split()
	if t == '1':
		a = int(a) - 1
		seg.set(a, d[b])
	else:
		a = int(a) - 1
		b = int(b) - 1
		print(seg.prod(a, b + 1)[3])