#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long mod = 1000000007;
const int MAXN = 150;
const int MAXL = 2000100;
int l, n;
char s1[MAXL], s2[MAXL], ans[MAXN];
struct edge {
  int from, to, cap, flow, cost;
};
inline int ID(char c) {
  if (c >= 'A' && c <= 'Z')
    return c - 'A' + 26;
  else
    return c - 'a';
}
inline char idtoc(int id) {
  if (id < 26) return 'a' + id;
  return 'A' + id - 26;
}
struct MCMF {
  int n, m, s, t;
  vector<edge> edges;
  vector<int> G[MAXN];
  int inq[MAXN], d[MAXN], p[MAXN], a[MAXN];
  void init(int n) {
    this->n = n;
    for (int i = 0; i < n; ++i) G[i].clear();
    edges.clear();
  }
  void add(int from, int to, int cap, int cost) {
    edges.push_back(edge{from, to, cap, 0, cost});
    edges.push_back(edge{to, from, 0, 0, -cost});
    m = edges.size();
    G[from].push_back(m - 2);
    G[to].push_back(m - 1);
  }
  bool BellmanFord(int s, int t, int &flow, int &cost) {
    memset(d, INF, sizeof(d));
    memset(inq, 0, sizeof(inq));
    d[s] = 0, inq[s] = 1, p[s] = 0, a[s] = INF;
    queue<int> q;
    q.push(s);
    while (!q.empty()) {
      int u = q.front();
      q.pop();
      inq[u] = 0;
      for (int i = 0; i < G[u].size(); ++i) {
        edge &e = edges[G[u][i]];
        if (e.cap > e.flow && d[e.to] > d[u] + e.cost) {
          d[e.to] = d[u] + e.cost;
          p[e.to] = G[u][i];
          a[e.to] = min(a[u], e.cap - e.flow);
          if (!inq[e.to]) {
            q.push(e.to);
            inq[e.to] = 1;
          }
        }
      }
    }
    if (d[t] == INF) return false;
    flow += a[t];
    cost += d[t] * a[t];
    for (int u = t; u != s; u = edges[p[u]].from) {
      edges[p[u]].flow += a[t];
      edges[p[u] ^ 1].flow -= a[t];
    }
    return true;
  }
  int MincostMaxflow(int s, int t, int &cost) {
    int flow = 0;
    cost = 0;
    while (BellmanFord(s, t, flow, cost))
      ;
    return flow;
  }
  void out() {
    m = edges.size();
    int k = (n - 2) >> 1;
    for (int i = 0; i < m; ++i) {
      edge &e = edges[i];
      if (e.flow == 1 && e.from != 0 && e.to != 2 * k + 1)
        ans[e.from - 1] = idtoc(e.to - k - 1);
    }
    printf("%s\n", ans);
  }
};
int g[MAXN][MAXN];
int main() {
  scanf("%d%d%s%s", &l, &n, s1, s2);
  MCMF G;
  G.init(n * 2 + 2);
  for (int i = 0; i < l; ++i) g[ID(s1[i])][ID(s2[i])]++;
  for (int i = 0; i < n; ++i) {
    G.add(0, i + 1, 1, 0);
    G.add(i + 1 + n, 2 * n + 1, 1, 0);
  }
  for (int i = 0; i < n; ++i)
    for (int j = 0; j < n; ++j) G.add(i + 1, j + n + 1, 1, -g[i][j]);
  int cost;
  G.MincostMaxflow(0, 2 * n + 1, cost);
  printf("%d\n", -cost);
  G.out();
  return 0;
}
