#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

struct node {
	int val[4][4];
};

node def;

template<class V,int NV> class SegTree_1 {
public:
	vector<V> val;
	V comp(V l,V r){
		V m;
		int i,j,k;
		FOR(j,4) FOR(i,j+1) m.val[i][j]=1<<20;
		FOR(i,4) for(j=i;j<4;j++) for(k=j;k<4;k++) m.val[i][k]=min(m.val[i][k],l.val[i][j]+r.val[j][k]);
		return m;
	};
	
	SegTree_1(){val=vector<V>(NV*2);};
	V getval(int x,int y,int l=0,int r=NV,int k=1) { // x<=i<y
		if(r<=x || y<=l) return def;
		if(x<=l && r<=y) return val[k];
		return comp(getval(x,y,l,(l+r)/2,k*2),getval(x,y,(l+r)/2,r,k*2+1));
	}
	void update(int entry, int v) {
		entry += NV;
		int i,j;
		FOR(j,4) FOR(i,j+1) val[entry].val[i][j]=(v<i||v>j);
		while(entry>1) entry>>=1, val[entry]=comp(val[entry*2],val[entry*2+1]);
	}
};
SegTree_1<node,1<<18> st;
int N,Q;
string S;

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>Q>>S;
	FOR(i,N) st.update(i,S[i]-'A');
	while(Q--) {
		cin>>i;
		if(i==1) {
			cin>>x>>S;
			st.update(x-1,S[0]-'A');
		}
		else {
			int L,R;
			cin>>L>>R;
			L--;
			auto ret=st.getval(L,R);
			cout<<ret.val[0][3]<<endl;
		}
	}
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}