import re

def nchars(s, n):
    """文字列 s に、同じ文字が n 個以上連続している部分文字列を見つける
    """
    assert n > 0
    reg = re.compile("(.)\\1{%d,}" % (n - 1))  # カンマを取ると n 個ちょうどになる
    while True:
        m = reg.search(s)
        if not m:
            break
        yield m.group(0)
        s = s[m.end():]

def main():
    [N, K] = [int(number) for number in input().split()]
    S = input()
    s = [len(rl) for rl in list(nchars(S,1))]
    i = K
    while (i > 0):
        len_s = len(s)
        if len_s < 3:
            break
        if i == K:
            three_range_arr = [s[j] + s[j+1] + s[j+2] for j in range(len_s) if j < len_s-2]
            max_tra = max(three_range_arr)
            index = three_range_arr.index(max_tra)
        else:
            index = s.index(max(s))
            if index == 0:
                max_tra = s[index]+s[index+1]+s[index+2]
            elif index == len_s - 1:
                max_tra = s[index-2] + s[index-1] + s[index]
                index -= 2
            else:
                if index == 1:
                    maxs = [s[index-1] + s[index] + s[index+1], s[index]+s[index+1]+s[index+2]]
                elif index == len_s-2:
                    maxs = [s[index-2] + s[index-1] + s[index], s[index-1] + s[index] + s[index+1]]
                else:
                    maxs = [s[index-2] + s[index-1] + s[index], s[index-1] + s[index] + s[index+1], s[index]+s[index+1]+s[index+2]]
                max_tra = max(maxs)
                offset = maxs.index(maxs) - 2
                if index == 1:
                    offset += 1
                index += offset
        if index == 0:
            ss = [max_tra]
            ss.extend(s[index+3:len_s])
        elif index == len_s - 3:
            ss = s[0:index-1]
            ss.extend([max_tra])
        else:
            ss = s[0:index-1]
            ss.extend([max_tra])
            ss.extend(s[index+3:len_s])
        #s = s[0:index-2].append(max_tra)#.extend(s[index+1, len_s-1])
        i-=1
        s=ss

    if i > 0:
        print(N-1)
    else:
        print(sum(s)-len(s))

if __name__ == '__main__':
    main()


### FIXED CODE ###

import re

def nchars(s, n):
    """文字列 s に、同じ文字が n 個以上連続している部分文字列を見つける
    """
    assert n > 0
    reg = re.compile("(.)\\1{%d,}" % (n - 1))  # カンマを取ると n 個ちょうどになる
    while True:
        m = reg.search(s)
        if not m:
            break
        yield m.group(0)
        s = s[m.end():]


def main():
    [N, K] = [int(number) for number in input().split()]
    S = input()
    s = [len(rl) for rl in list(nchars(S,1))]
    happy = sum(s)-len(s) + K * 2
    if happy >= N:
        print(N-1)
    else:
        print(happy)


    """
    i = K
    while (i > 0):
        len_s = len(s)
        if len_s < 3:
            break
        if i == K:
            three_range_arr = [s[j] + s[j+1] + s[j+2] for j in range(len_s) if j < len_s-2]
            max_tra = max(three_range_arr)
            index = three_range_arr.index(max_tra)
        else:
            index = s.index(max(s))
            if index == 0:
                max_tra = s[index]+s[index+1]+s[index+2]
            elif index == len_s - 1:
                max_tra = s[index-2] + s[index-1] + s[index]
                index -= 2
            else:
                if index == 1:
                    maxs = [s[index-1] + s[index] + s[index+1], s[index]+s[index+1]+s[index+2]]
                elif index == len_s-2:
                    maxs = [s[index-2] + s[index-1] + s[index], s[index-1] + s[index] + s[index+1]]
                else:
                    maxs = [s[index-2] + s[index-1] + s[index], s[index-1] + s[index] + s[index+1], s[index]+s[index+1]+s[index+2]]
                max_tra = max(maxs)
                offset = maxs.index(maxs) - 2
                if index == 1:
                    offset += 1
                index += offset
        if index == 0:
            ss = [max_tra]
            ss.extend(s[index+3:len_s])
        elif index == len_s - 3:
            ss = s[0:index-1]
            ss.extend([max_tra])
        else:
            ss = s[0:index-1]
            ss.extend([max_tra])
            ss.extend(s[index+3:len_s])
        #s = s[0:index-2].append(max_tra)#.extend(s[index+1, len_s-1])
        i-=1
        s=ss

    if i > 0:
        print(N-1)
    else:
        print(sum(s)-len(s))
    """


if __name__ == '__main__':
    main()