import java.math.*;
import java.util.*;

public class Main {
	static boolean check(int ka,int kb,StringBuffer test){
		for(int i=0;i<test.length();i++){
			if(test.charAt(i)=='R'){
				if(i+ka<test.length() && test.charAt(i+ka)=='R') return false;
			}else if(test.charAt(i)=='B'){
				if(i+kb<test.length() && test.charAt(i+kb)=='B')return false;
			}
		}
		return true;
	}
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int ka = sc.nextInt();
		int kb = sc.nextInt();
		String ss=sc.next();
		int[] block = new int[20];
		int temp=0;
		for(int i=29;i>=0;i--){
			if(ss.charAt(i)=='R' || ss.charAt(i)=='B' ){
				block[temp]=i;
				temp++;
			}
		}
		int ans=Integer.MAX_VALUE;
		for(int i=0;i< (1<<21) ;i++){
			if(Integer.bitCount(i)>=ans) continue;
			StringBuffer test = new StringBuffer(ss);
			for(int j=0;j<20;j++){
				if( ( (i>>j) &1) ==1 ){
					test.deleteCharAt(block[j]);
				}
			}
			if(check(ka,kb,test)){
				ans=Integer.bitCount(i);
			}
		}
		System.out.println(30-ans);
	}
}