from functools import cache
import bisect
import sys

base_limit = sys.getrecursionlimit()

def sc(cons):
    return (cons*(cons+1))/2

def brutetop(a, b, k):
    first = {}
    for i in range(len(a)):
        if a[i] not in first and a[i] != b[i]:
            first[a[i]] = i
    @cache
    def brute(Q, i, cons):
        if i == len(a):
            return sc(cons)

        if a[i] == b[i] or a[i] in Q:
            return brute(Q, i+1, cons+1)
        
        if len(Q) < k and (a[i] in first and i == first[a[i]]):
            Qcop = list(Q)
            bisect.insort(Qcop, a[i])

            res1 = brute(Q, i+1, 0) + sc(cons)
            res2 = brute(tuple(Qcop), i+1, cons+1)

            return max(res1, res2)
        
        return brute(Q, i+1, 0) + sc(cons)

    return brute((), 0, 0)

def main():
    ct = int(input())
    ress = []

    for _ in range(ct):
        # len(a, b), char lim
        n, k = [int(i) for i in input().split()]
        a = input() # a = base string
        b = input() # b = objective

        sys.setrecursionlimit(max(len(a), base_limit))
        print(int(brutetop(a, b, k)))
        #ress.append(str(int(brutetop(a, b, k))))
    #print("\n".join(ress))
        


main()