#include <bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include <atcoder/all>
using namespace std;
using namespace atcoder;

struct S {
  int a00, a01, a02, a03, a11, a12, a13, a22, a23, a33;
};
S op(S A, S B) {
  return S{
      A.a00 + B.a00,
      min(A.a00 + B.a01, A.a01 + B.a11),
      min(A.a00 + B.a02, min(A.a01 + B.a12, A.a02 + B.a22)),
      min(A.a00 + B.a03, min(A.a01 + B.a13, min(A.a02 + B.a23, A.a03 + B.a33))),
      A.a11 + B.a11,
      min(A.a11 + B.a12, A.a12 + B.a22),
      min(A.a11 + B.a13, min(A.a12 + B.a23, A.a13 + B.a33)),
      A.a22 + B.a22,
      min(A.a22 + B.a23, A.a23 + B.a33),
      A.a33 + B.a33};
}
S e() { return S{}; }

}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n, q;
  cin >> n >> q;
  string s;
  cin >> s;
  vector<S> init(n);
  S c2mat[]{S{0, 0, 0, 0, 1, 1, 1, 1, 1, 1}, S{1, 0, 0, 0, 0, 0, 0, 1, 1, 1},
            S{1, 1, 0, 0, 1, 0, 0, 0, 0, 1}, S{1, 1, 1, 0, 1, 1, 0, 1, 0, 0}};
  segtree<S, op, e> seg([&]() {
    vector<S> init(n);
    for (int i = 0; i < n; i++) init[i] = c2mat[s[i] - 'A'];
    return init;
  }());
  while (q--) {
    int type;
    cin >> type;
    if (type == 1) {
      int x;
      char c;
      cin >> x >> c;
      seg.set(x - 1, c2mat[c - 'A']);
    } else {
      int l, r;
      cin >> l >> r;
      cout << seg.prod(l - 1, r).a03 << '\n';
    }
  }
}