import sys


def main(argv):
    path = argv[0]
    f = open(path)
    n, k = map(int, f.readline().split())
    s = f.readline().rstrip()

    if n <= 3:
        print(n)
        return

    cnt = [0] * (n - 2)
    cnt[0] += int(s[0] == "S") + int(s[1] == "S")
    cnt[1] += int(s[1] == "S")
    cnt[-1] += int(s[-1] == "S") + int(s[-2] == "S")
    cnt[-2] += int(s[-2] == "S")
    for i in range(2, n - 2):
        m = int(s[i] == "S")
        cnt[i - 2] += m
        cnt[i - 1] += m
        cnt[i] += m

    first_sum = sum([c for c in cnt[::3]])
    second_sum = sum([c for c in cnt[1::3]])
    third_sum = sum([c for c in cnt[2::3]])
    idx_search_order = []
    if first_sum >= second_sum >= third_sum:
        idx_search_order = [0, 1, 2]
    elif first_sum >= third_sum >= second_sum:
        idx_search_order = [0, 2, 1]
    elif second_sum >= first_sum >= third_sum:
        idx_search_order = [1, 0, 2]
    elif second_sum >= third_sum >= first_sum:
        idx_search_order = [1, 2, 0]
    elif third_sum >= first_sum >= second_sum:
        idx_search_order = [2, 0, 1]
    else:
        idx_search_order = [2, 1, 0]

    cnt_i = {0: set(), 1: set(), 2: set(), 3: set()}
    for i, c in enumerate(cnt):
        cnt_i[c].add(i)

    ans = s.count(".")
    for c in range(3, 0, -1):
        while cnt_i[c] and k > 0:
            for offset in idx_search_order:
                this_cnt_i = cnt_i[c] & {i for i in range(offset, n - 2, 3)}
                while this_cnt_i:
                    i = this_cnt_i.pop()
                    this_cnt_i -= {i - 2, i - 1, i, i + 1, i + 2}
                    cnt_i[c] -= {i - 2, i - 1, i, i + 1, i + 2}
                    ans += c
                    k -= 1
    print(ans)


if __name__ == "__main__":
    main(sys.argv[1:])