#include <bits/stdc++.h>
using namespace std;

#define rep(i, l, r) for (int i = (int)(l); i<(int)(r); i++)
#define ll long long

int main() {
    int N, Q; cin >> N >> Q;
    string S; cin >> S;
    while(Q--) {
        int q; cin >> q;
        if (q == 1) {
            int x; char c; cin >> x >> c;
            x--;
            S[x] = c;
        }
        else {
            int l, r; cin >> l >> r;
            l--;
            vector<vector<int>> dp(r-l+1, vector<int>(4, N+1));
            dp[0][0] = 0;
            rep(i, 0, r-l) {
                rep(j, 0, 4) {
                    rep(k, j, 4) {
                        dp[i+1][k] = min(dp[i+1][k], dp[i][j] + (S[l+i]-'A' == k? 0 : 1));
                    }
                }
            }
            cout << min(min(dp.back()[0], dp.back()[1]), min(dp.back()[2], dp.back()[3])) << endl;
        }
    }
}