#include <bits/stdc++.h>
using namespace std;
const int N = 55;
int decyph[300], revdecyph[N], K;
int wag[N][N];
bool G[N][N], vis[2 * N], pok[2 * N];
int mate[2 * N];
char slo[2][2000 * 1000 + 3];
queue<int> kol;
void pokrycie() {
  for (int i = 0; i < 2 * K; i++) vis[i] = pok[i] = 0;
  for (int i = 0; i < 2 * K; i++)
    if (mate[i] == -1) {
      vis[i] = 1;
      kol.push(i);
    }
  while (!kol.empty()) {
    int x = kol.front();
    kol.pop();
    int d;
    if (x < K)
      d = K;
    else
      d = 0;
    if (mate[x] == -1 || vis[mate[x]])
      for (int i = 0; i < K; i++) {
        if (d == 0) {
          if (G[i][x % K] && vis[i + d] == 0) {
            vis[i + d] = 1;
            kol.push(i + d);
          }
        } else {
          if (G[x % K][i] && vis[i + d] == 0) {
            vis[i + d] = 1;
            kol.push(i + d);
          }
        }
      }
    if (mate[x] > -1 && vis[mate[x]] == 0) {
      pok[x] = 1;
      vis[mate[x]] = 1;
      kol.push(mate[x]);
    }
  }
  for (int i = 0; i < K; i++)
    if (vis[i] == 0 && mate[i] > -1 && vis[mate[i]] == 0) pok[i] = 1;
}
bool dfs(int x) {
  if (vis[x]) return 0;
  vis[x] = 1;
  for (int i = 0; i < K; i++)
    if (G[x][i])
      if (mate[i + K] == -1 || dfs(mate[i + K])) {
        mate[x] = i + K;
        mate[i + K] = x;
        return 1;
      }
  return 0;
}
int main() {
  int n;
  scanf("%d%d", &n, &K);
  scanf("%s%s", slo[0], slo[1]);
  for (int i = 'a'; i <= 'z'; i++) {
    revdecyph[i - 'a'] = i;
    decyph[i] = i - 'a';
  }
  for (int i = 'A'; i <= 'Z'; i++) {
    revdecyph['z' - 'a' + 1 + i - 'A'] = i;
    decyph[i] = 'z' - 'a' + 1 + i - 'A';
  }
  for (int i = 0; i < n; i++) wag[decyph[slo[0][i]]][decyph[slo[1][i]]]++;
  for (int i = 0; i < K; i++)
    for (int j = 0; j < K; j++) wag[i][j] = n - wag[i][j];
  for (int i = 0; i < K; i++) mate[i] = mate[i + K] = -1;
  for (int i = 0; i < K; i++) {
    int war = wag[i][0];
    for (int j = 0; j < K; j++) war = min(war, wag[i][j]);
    for (int j = 0; j < K; j++) wag[i][j] -= war;
  }
  for (int i = 0; i < K; i++) {
    int war = wag[0][i];
    for (int j = 0; j < K; j++) war = min(war, wag[j][i]);
    for (int j = 0; j < K; j++) wag[j][i] -= war;
  }
  while (1) {
    for (int i = 0; i < K; i++)
      for (int j = 0; j < K; j++) {
        if (wag[i][j] == 0)
          G[i][j] = 1;
        else
          G[i][j] = 0;
      }
    bool nadal = true;
    while (nadal) {
      for (int i = 0; i < K; i++) vis[i] = 0;
      nadal = false;
      for (int x = 0; x < K; x++)
        if (mate[x] == -1 && dfs(x)) nadal = 1;
    }
    for (int i = 0; i < K; i++)
      if (mate[i] == -1) nadal = true;
    if (nadal == 0) break;
    pokrycie();
    int war = n;
    for (int i = 0; i < K; i++)
      for (int j = 0; j < K; j++)
        if (pok[i] == 0 && pok[j + K] == 0) war = min(war, wag[i][j]);
    for (int i = 0; i < K; i++)
      for (int j = 0; j < K; j++) {
        if (pok[i] == 1 && pok[j + K] == 1) wag[i][j] += war;
        if (pok[i] == 0 && pok[j + K] == 0) wag[i][j] -= war;
      }
  }
  int wynik = 0;
  for (int i = 0; i < n; i++) {
    slo[0][i] = revdecyph[mate[decyph[slo[0][i]]] - K];
    if (slo[0][i] == slo[1][i]) wynik++;
  }
  printf("%d\n", wynik);
  for (int i = 0; i < K; i++) printf("%c", revdecyph[mate[i] - K]);
  printf("\n");
}
