#include <bits/stdc++.h>
using namespace std;
const int N = 1e7 + 20, LOG = 30, MODM = 1e9 + 7;
const int COL = 1e6 + 5;
const long long INF = 1e18 + 7;
const double EPS = 1e-12;
string base;
unordered_map<char, int> num;
void ini() {
  for (char c = 'a'; c <= 'z'; ++c) {
    base += c;
    num[c] = base.size();
  }
  for (char c = 'A'; c <= 'Z'; ++c) {
    base += c;
    num[c] = base.size();
  }
}
const int MAXN = 100;
int a[MAXN][MAXN];
int u[MAXN], v[MAXN], p[MAXN], way[MAXN];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  string s1, s2;
  cin >> s1 >> s2;
  ini();
  for (int i = 0; i < n; ++i) {
    --a[num[s1[i]]][num[s2[i]]];
  }
  for (int i = 1; i <= k; ++i) {
    p[0] = i;
    int pos_j = 0;
    vector<int> min_v(k + 1, MODM);
    vector<bool> used(k + 1, false);
    do {
      used[pos_j] = true;
      int ind1 = p[pos_j], delta = MODM, pos_j2;
      for (int j = 1; j <= k; ++j) {
        if (!used[j]) {
          int current = a[ind1][j] - u[ind1] - v[j];
          if (current < min_v[j]) {
            min_v[j] = current;
            way[j] = pos_j;
          }
          if (min_v[j] < delta) {
            delta = min_v[j];
            pos_j2 = j;
          }
        }
      }
      for (int j = 0; j <= k; ++j) {
        if (used[j]) {
          u[p[j]] += delta;
          v[j] -= delta;
        } else {
          min_v[j] -= delta;
        }
      }
      pos_j = pos_j2;
    } while (p[pos_j] != 0);
    do {
      int jj = way[pos_j];
      p[pos_j] = p[jj];
      pos_j = jj;
    } while (pos_j);
  }
  vector<int> ans(k + 1);
  for (int j = 1; j <= k; ++j) ans[p[j]] = j;
  cout << v[0] << "\n";
  for (auto to : ans)
    if (to != 0) cout << base[to - 1];
  return 0;
}
