import sys

from munkres import Munkres

m = Munkres()

n, k = map(int, input().split())

ciphering = input()
sol = input()

def from_letter(a):
    if ord(a) < ord('a'):
        return 26+ord(a)-ord('A')
    return ord(a)-ord('a')

def to_letter(n):
    if n > 25:
        return chr(n-26+ord('A'))
    return chr(n+ord('a'))

mat = [
    [0]*k for _ in range(k)
]

for letter1, letter2 in zip(ciphering, sol):
    mat[from_letter(letter1)][from_letter(letter2)] += 1


cost_matrix = []
for row in mat:
    cost_row = []
    for col in row:
        cost_row += [sys.maxsize - col]
    cost_matrix += [cost_row]

m = Munkres()
indexes = m.compute(cost_matrix)
total = 0
for row, column in indexes:
    value = mat[row][column]
    total += value

print(total)

for row, column in indexes:
    print(to_letter(column), end='')
print()