#include <bits/stdc++.h>
using namespace std;
int n, k;
int T[60][60], u[60], v[60], p[60], way[60];
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cin >> n >> k;
  string a, b;
  cin >> a >> b;
  for (int i = 0; i < n; i++) {
    int aa, bb;
    if (a[i] < 'a') {
      aa = 26 + a[i] - 'A' + 1;
    } else {
      aa = a[i] - 'a' + 1;
    }
    if (b[i] < 'a') {
      bb = 26 + b[i] - 'A' + 1;
    } else {
      bb = b[i] - 'a' + 1;
    }
    T[aa][bb]--;
  }
  int res = 0;
  for (int i = 1; i <= k; i++) {
    p[0] = i;
    int cf = 0;
    vector<int> min_value(k + 1, 1e9);
    vector<bool> used(k + 1, false);
    used[cf] = true;
    int ic = p[cf], j2;
    int heh = 1e9;
    for (int j = 1; j <= k; j++) {
      if (!used[j]) {
        int value = T[ic][j] - u[ic] - v[j];
        if (value < min_value[j]) {
          min_value[j] = value;
          way[j] = cf;
        }
        if (min_value[j] < heh) {
          heh = min_value[j];
          j2 = j;
        }
      }
    }
    for (int j = 0; j <= k; j++) {
      if (used[j]) {
        u[p[j]] += heh;
        v[j] -= heh;
      } else {
        min_value[j] -= heh;
      }
    }
    cf = j2;
    while (p[cf]) {
      used[cf] = true;
      int ic = p[cf], j2;
      int heh = 1e9;
      for (int j = 1; j <= k; j++) {
        if (!used[j]) {
          int value = T[ic][j] - u[ic] - v[j];
          if (value < min_value[j]) {
            min_value[j] = value;
            way[j] = cf;
          }
          if (min_value[j] < heh) {
            heh = min_value[j];
            j2 = j;
          }
        }
      }
      for (int j = 0; j <= k; j++) {
        if (used[j]) {
          u[p[j]] += heh;
          v[j] -= heh;
        } else {
          min_value[j] -= heh;
        }
      }
      cf = j2;
    }
    int g = way[cf];
    p[cf] = p[g];
    cf = g;
    while (cf) {
      int g = way[cf];
      p[cf] = p[g];
      cf = g;
    }
    res = max(res, v[0]);
  }
  vector<int> ans(k + 1);
  for (int i = 1; i <= k; i++) ans[p[i]] = i;
  cout << res << '\n';
  for (int i = 1; i <= k; i++) {
    if (ans[i] <= 26) {
      cout << (char)('a' - 1 + ans[i]);
    } else {
      cout << (char)('A' + 5 + ans[i] - 32);
    }
  }
}
