using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("1 10");
            WillReturn.Add("RRRRRRRRRRWWWWWWWWWWBBBBBBBBBB");
            //21
            //Rの1個となりがRであってはならないのでRを9個抜く。
            //Bの10個となりにBがあることは無いのでBは抜かなくてよい。
            //最終的には RWWWWWWWWWWBBBBBBBBBB が残る。
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("1 2");
            WillReturn.Add("WRWWBBRRRWWBBRRRRWWWBBBRRWWBBB");
            //22
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("7 11");
            WillReturn.Add("BWBWRWRRWBRRWRRWWBBBRRWBBRWBBW");
            //27
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        var sw = System.Diagnostics.Stopwatch.StartNew();

        List<string> InputList = GetInputList();
        int[] wkArr = InputList[0].Split(' ').Select(X => int.Parse(X)).ToArray();
        int Kr = wkArr[0];
        int Kb = wkArr[1];
        string S = InputList[1];

        Console.WriteLine(ExecDFS(S, Kr, Kb));
    }

    struct JyoutaiDef
    {
        internal int CurrP;
        internal string CurrStr;
        internal int RemoveCnt;
    }

    //2の20乗通りの順列を列挙
    static int ExecDFS(string pS, int pKr, int pKb)
    {
        var stk = new Stack<JyoutaiDef>();
        JyoutaiDef WillPush;
        WillPush.CurrP = 0;
        WillPush.CurrStr = "";
        WillPush.RemoveCnt = 0;
        stk.Push(WillPush);

        int AnswerRemoveCnt = int.MaxValue;

        while (stk.Count > 0) {
            JyoutaiDef Popped = stk.Pop();

            //クリア判定
            if (Popped.CurrP > pS.Length - 1) {
                if (IsOK(Popped.CurrStr, pKr, pKb)) {
                    if (AnswerRemoveCnt > Popped.RemoveCnt) {
                        AnswerRemoveCnt = Popped.RemoveCnt;
                    }
                }
                continue;
            }

            Action<string> PushSyori = pAddStr =>
            {
                WillPush.CurrP = Popped.CurrP + 1;
                WillPush.CurrStr = Popped.CurrStr + pAddStr;
                WillPush.RemoveCnt = Popped.RemoveCnt;
                if (pAddStr.Length == 0) WillPush.RemoveCnt++;

                //下限値枝切り
                if (AnswerRemoveCnt <= WillPush.RemoveCnt)
                    return;

                stk.Push(WillPush);
            };

            if (pS[Popped.CurrP] == 'B') {
                PushSyori("B"); PushSyori("");
            }
            if (pS[Popped.CurrP] == 'R') {
                PushSyori("R"); PushSyori("");
            }
            if (pS[Popped.CurrP] == 'W') {
                PushSyori("W");
            }
        }
        return pS.Length - AnswerRemoveCnt;
    }

    //OKなレンガの配置かを判定
    static bool IsOK(string pS, int pKr, int pKb)
    {
        int UB = pS.Length - 1;
        for (int I = 0; I <= UB; I++) {
            if (pS[I] == 'R' && I + pKr <= UB && pS[I + pKr] == 'R')
                return false;
            if (pS[I] == 'B' && I + pKb <= UB && pS[I + pKb] == 'B')
                return false;
        }
        return true;
    }
}