#include<bits/stdc++.h>
using namespace std;
#include<atcoder/segtree>
#define q int
#define c(i,v,n)for(q i=v;i<n;i++)
struct S{q D[4][4];S(){c(i,0,4)c(j,0,4)D[i][j]=0;}S(q x){c(i,0,4)c(j,0,4)D[i][j]=i>x||x>j;}};
S o(S L,S R){S K;c(i,0,4)c(j,i,4){K.D[i][j]=1e9;c(k,i,j+1)K.D[i][j]=min(K.D[i][j],L.D[i][k]+R.D[k][j]);}return K;}
S e(){return S();}
int main(){
 q N,Q,p,l;string T;cin>>N>>Q>>T;
 vector<S>A(N);c(i,0,N)A[i]=S(T[i]-'A');
 atcoder::segtree<S,o,e>g(A);
 while(Q--){
 	cin>>p;
 	if(--p){
 		cin>>p>>l;
 		cout<<g.prod(p-1,l).D[0][3]<<endl;
 	}
 	else {
 		char c;cin>>l>>c;
 		g.set(l-1,S(c-'A'));
 	}
 }
}