using Lib;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using static Lib.OutputLib;
public class Solver
{
    const bool MultiTestCase = false;
    void Solve()
    {
        int n = ri, q = ri;
        var s = rs;

        var data = new S[n];
        for (int i = 0; i < n; i++) data[i] = new S(s[i] - 'A');
        var seg = new SegTree<S, Op>(data);
        while (q-- > 0)
        {
            if (ri == 1)
            {
                seg.Set(ri - 1, new S(rs[0] - 'A'));
            }
            else
            {
                var a = seg.Prod(ri - 1, ri).a;
                int ans = IINF;
                for (int i = 0; i < 4; i++)
                    for (int j = i; j < 4; j++)
                    {
                        int k = 4 * i + j;
                        if (a[k] < ans) ans = a[k];
                    }
                Write(ans);
            }
        }
    }

    struct S
    {
        public int[] a;
        public S()
        {
            a = new int[16];
        }
        public S(int v)
        {
            a = new int[16];
            Array.Fill(a, 1);
            for (int i = 0; i <= v; i++)
                for (int j = v; j < 4; j++)
                    a[4 * i + j] = 0;
        }
    }

    struct Op : IMonoid<S>
    {
        public S Identity => new S();
        public S Operate(S x, S y)
        {
            var z = new S();
            for (int i = 0; i < 4; i++)
                for (int j = i; j < 4; j++)
                {
                    int v = IINF;
                    for (int k = i; k <= j; k++)
                    {
                        int w = x.a[4 * i + k] + y.a[4 * k + j];
                        if (w < v) v = w;
                    }
                    z.a[4 * i + j] = v;
                }
            return z;
        }
    }

#pragma warning disable CS0162
    public Solver() { if (!MultiTestCase) Solve(); else for (int t = ri; t > 0; t--) Solve(); }
#pragma warning restore CS0162

    const int IINF = 1 << 30;
    const long INF = 1L << 60;
    int ri { [MethodImpl(256)] get => (int)sc.Integer(); }
    long rl { [MethodImpl(256)] get => sc.Integer(); }
    uint rui { [MethodImpl(256)] get => (uint)sc.UInteger(); }
    ulong rul { [MethodImpl(256)] get => sc.UInteger(); }
    double rd { [MethodImpl(256)] get => sc.Double(); }
    string rs { [MethodImpl(256)] get => sc.Scan(); }
    string rline { [MethodImpl(256)] get => sc.Line(); }
    public StreamScanner sc = new StreamScanner(Console.OpenStandardInput());
    void ReadArray(out int[] a, int n) { a = new int[n]; for (int i = 0; i < a.Length; i++) a[i] = ri; }
    void ReadArray(out long[] a, int n) { a = new long[n]; for (int i = 0; i < a.Length; i++) a[i] = rl; }
    void ReadArray<T>(out T[] a, int n, Func<T> read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(); }
    void ReadArray<T>(out T[] a, int n, Func<int, T> read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(i); }
}

static class Program
{
    static public void Main(string[] args)
    {
        SourceExpander.Expander.Expand();
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver();
        Console.Out.Flush();
    }
}
#region Expanded by https://github.com/kzrnm/SourceExpander
namespace Lib{public class SegTree<T,Op>where Op:struct,IMonoid<T>{private static readonly Op op=default;public int Length{get;}protected readonly int log,size;public readonly T[]d;public SegTree(int sz){Length=sz;log=0;while((1<<log)<Length)log++;size=1<<log;d=new T[2*size];Array.Fill(d,op.Identity);}public SegTree(T[]v):this(v.Length){for(int i=0;i<Length;i++)d[size+i]=v[i];for(int i=size-1;i>0;i--)Update(i);}[MethodImpl(256)]internal void SetWithoutUpdate(int p,T v)=>d[p+size]=v;[MethodImpl(256)]internal void AllUpdate(){for(int i=size-1;i>0;i--)Update(i);}[MethodImpl(256)]private void Update(int p)=>d[p]=op.Operate(d[2*p],d[2*p+1]);[MethodImpl(256)]public T Get(int p)=>d[p+size];[MethodImpl(256)]public void Set(int p,T v){p+=size;d[p]=v;for(int i=1;i<=log;i++)Update(p>>i);}[MethodImpl(256)]public void Apply(int p,T v){p+=size;d[p]=op.Operate(d[p],v);for(int i=1;i<=log;i++)Update(p>>i);}public T AllProd()=>d[1];[MethodImpl(256)]public T Prod(int l,int r){T sl=op.Identity,sr=op.Identity;l+=size;r+=size;while(l<r){if((l&1)!=0)sl=op.Operate(sl,d[l++]);if((r&1)!=0)sr=op.Operate(d[ --r],sr);l>>=1;r>>=1;}return op.Operate(sl,sr);}[MethodImpl(256)]public int MaxRight(int l,Predicate<T>f){if(l==Length)return Length;l+=size;var s=op.Identity;do{while(l%2==0)l>>=1;if(!f(op.Operate(s,d[l]))){while(l<size){l<<=1;if(f(op.Operate(s,d[l]))){s=op.Operate(s,d[l]);l++;}}return l-size;}s=op.Operate(s,d[l]);l++;}while((l&-l)!=l);return Length;}[MethodImpl(256)]public int MinLeft(int r,Predicate<T>f){if(r==0)return 0;r+=size;var s=op.Identity;do{r--;while(r>1&&(r%2)!=0)r>>=1;if(!f(op.Operate(d[r],s))){while(r<size){r=(2*r+1);if(f(op.Operate(d[r],s))){s=op.Operate(d[r],s);r--;}}return r+1-size;}s=op.Operate(d[r],s);}while((r&-r)!=r);return 0;}}}
namespace Lib{public interface IMonoid<T>:IOperation<T>,IIdentity<T>{}public struct IntMinMonoid:IMonoid<int>{public int Identity=>int.MaxValue;public int Operate(int x,int y)=>Math.Min(x,y);}public struct IntMaxMonoid:IMonoid<int>{public int Identity=>int.MinValue;public int Operate(int x,int y)=>Math.Max(x,y);}public struct LongMinMonoid:IMonoid<long>{public long Identity=>long.MaxValue;public long Operate(long x,long y)=>Math.Min(x,y);}public struct LongMaxMonoid:IMonoid<long>{public long Identity=>long.MinValue;public long Operate(long x,long y)=>Math.Max(x,y);}public struct PairMonoid<T1,Op1,T2,Op2>:IMonoid<(T1,T2)>where Op1:struct,IMonoid<T1>where Op2:struct,IMonoid<T2>{public readonly static Op1 op1=default;public readonly static Op2 op2=default;public(T1,T2)Identity=>(op1.Identity,op2.Identity);public(T1,T2)Operate((T1,T2)x,(T1,T2)y)=>(op1.Operate(x.Item1,y.Item1),op2.Operate(x.Item2,y.Item2));}public struct Tuple2Monoid<T,Op>:IMonoid<(T,T)>where Op:struct,IMonoid<T>{public readonly static Op op=default;public(T,T)Identity=>(op.Identity,op.Identity);public(T,T)Operate((T,T)x,(T,T)y)=>(op.Operate(x.Item1,y.Item1),op.Operate(x.Item2,y.Item2));}public struct Tuple3Monoid<T,Op>:IMonoid<(T,T,T)>where Op:struct,IMonoid<T>{public readonly static Op op=default;public(T,T,T)Identity=>(op.Identity,op.Identity,op.Identity);public(T,T,T)Operate((T,T,T)x,(T,T,T)y)=>(op.Operate(x.Item1,y.Item1),op.Operate(x.Item2,y.Item2),op.Operate(x.Item3,y.Item3));}public struct Tuple4Monoid<T,Op>:IMonoid<(T,T,T,T)>where Op:struct,IMonoid<T>{public readonly static Op op=default;public(T,T,T,T)Identity=>(op.Identity,op.Identity,op.Identity,op.Identity);public(T,T,T,T)Operate((T,T,T,T)x,(T,T,T,T)y)=>(op.Operate(x.Item1,y.Item1),op.Operate(x.Item2,y.Item2),op.Operate(x.Item3,y.Item3),op.Operate(x.Item4,y.Item4));}}
namespace Lib{public interface IIdentity<T>{T Identity{get;}}}
namespace Lib{public interface IOperation<T>{T Operate(T x,T y);}}
namespace Lib{public partial class StreamScanner{public StreamScanner(Stream stream){str=stream;}private readonly Stream str;private readonly byte[]buf=new byte[1024];private int len,ptr;public bool isEof=false;public bool IsEndOfStream{get{return isEof;}}[MethodImpl(256)]private byte Read(){if(isEof)throw new EndOfStreamException();if(ptr>=len){ptr=0;if((len=str.Read(buf,0,1024))<=0){isEof=true;return 0;}}return buf[ptr++];}[MethodImpl(256)]public char Char(){byte b;do b=Read();while(b<33||126<b);return(char)b;}[MethodImpl(256)]public string Line(){var sb=new StringBuilder();for(var b=Char();b!=10&&!isEof;b=(char)Read())sb.Append(b);return sb.ToString();}[MethodImpl(256)]public string Scan(){var sb=new StringBuilder();for(var b=Char();b>=33&&b<=126;b=(char)Read())sb.Append(b);return sb.ToString();}[MethodImpl(256)]public long Integer(){long ret=0;var ng=false;byte b;do b=Read();while(b!='-'&&(b<'0'||'9'<b));if(b=='-'){ng=true;b=Read();}for(;'0'<=b&&b<='9';b=Read())ret=ret*10+(b^'0');return ng?-ret:ret;}[MethodImpl(256)]public ulong UInteger(){ulong ret=0;byte b;do b=Read();while(b<'0'||'9'<b);for(;'0'<=b&&b<='9';b=Read())ret=ret*10+(ulong)(b^'0');return ret;}[MethodImpl(256)]public double Double()=>double.Parse(Scan());}}
namespace Lib{public static class OutputLib{[MethodImpl(256)]public static void WriteJoin<T>(string s,IEnumerable<T>t)=>Console.WriteLine(string.Join(s,t));[MethodImpl(256)]public static void WriteMat<T>(T[,]a,string sep=" "){int sz1=a.GetLength(0),sz2=a.GetLength(1);var b=new T[sz2];for(int i=0;i<sz1;i++){for(int j=0;j<sz2;j++)b[j]=a[i,j];WriteJoin(sep,b);}}[MethodImpl(256)]public static void WriteMat<T>(T[][]a,string sep=" "){foreach(var ar in a)WriteJoin(sep,ar);}[MethodImpl(256)]public static void WriteMat<T>(T[][]a,Func<T,string>map,string sep=" "){foreach(var ar in a)WriteJoin(sep,ar.Select(x=>map(x)));}[MethodImpl(256)]public static void Write(object t)=>Console.WriteLine(t.ToString());[MethodImpl(256)]public static void Write(params object[]arg)=>Console.WriteLine(string.Join(" ",arg.Select(x=>x.ToString())));[MethodImpl(256)]public static void Write(string str)=>Console.WriteLine(str);[MethodImpl(256)]public static void WriteFlush(object t){Console.WriteLine(t.ToString());Console.Out.Flush();}[MethodImpl(256)]public static void WriteError(object t)=>Console.Error.WriteLine(t.ToString());[MethodImpl(256)]public static void Flush()=>Console.Out.Flush();[MethodImpl(256)]public static void YN(bool t)=>Console.WriteLine(t?"YES":"NO");[MethodImpl(256)]public static void Yn(bool t)=>Console.WriteLine(t?"Yes":"No");[MethodImpl(256)]public static void yn(bool t)=>Console.WriteLine(t?"yes":"no");[MethodImpl(256)]public static void DeleteLine()=>Console.Write("\x1b[1A\x1b[2K");[MethodImpl(256)]public static void ProgressBar(long now,long total,int blocks=50){int x=(int)((2*now*blocks+1)/(2*total));Console.Write($"\x1b[G[\x1b[42m{string.Concat(Enumerable.Repeat("_",x))}\x1b[0m{string.Concat(Enumerable.Repeat("_",blocks-x))}] : {now} / {total}");}}}
namespace SourceExpander{public class Expander{[Conditional("EXP")]public static void Expand(string inputFilePath=null,string outputFilePath=null,bool ignoreAnyError=true){}public static string ExpandString(string inputFilePath=null,bool ignoreAnyError=true){return "";}}}
#endregion Expanded by https://github.com/kzrnm/SourceExpander