from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
# input = sys.stdin.readline

Kr,Kb = map(int,input().split())
S = list(input())
A = []
for s in S:
    if s=='R':
        A.append(2)
    elif s=='B':
        A.append(1)
    else:
        A.append(0)

R = []
B = []
for i,s in enumerate(S):
    if s=='R':
        R.append(i)
    if s=='B':
        B.append(i)
S = A[:]

def is_ok(X):
    
    T = []
    for i,s in enumerate(S):
        if i in X:
            continue
        T.append(s)
    
    if any((T[i]==1)&(T[i]==T[i+Kb]) for i in range(len(T)-Kb)):
        return -1
        
    if any((T[i]==2)&(T[i]==T[i+Kr]) for i in range(len(T)-Kr)):
        return -1
        
    return len(T)
   
Z = R+B
ans = -1

for i in range(1<<20):
    
    
    X = []
    
    for j in range(20):
        
        if (i>>j)&1:
            X.append(Z[j])
        
    X = set(X)
    ans = max(ans,is_ok(X))
print(ans)