#include <bits/stdc++.h>
using namespace std;
int n, m, g[52][52];
int HASH(char i) {
  if ('a' <= i && i <= 'z') return i - 'a';
  if ('A' <= i && i <= 'Z') return 26 + i - 'A';
}
char HASH(int i) {
  if (i < 26) return 'a' + i;
  if (i < 52) return 'A' + i - 26;
}
char s[2001000], t[2001000];
namespace MCMF {
const int N = 200, M = 200000;
int head[N], cnt, dis[N], fr[N], id[N], S, T, cost;
struct node {
  int to, next, val, cost;
} edge[M];
void ae(int u, int v, int w, int c) {
  edge[cnt].cost = c, edge[cnt].next = head[u], edge[cnt].to = v,
  edge[cnt].val = w, head[u] = cnt++;
  edge[cnt].cost = -c, edge[cnt].next = head[v], edge[cnt].to = u,
  edge[cnt].val = 0, head[v] = cnt++;
}
queue<int> q;
bool in[N];
bool SPFA() {
  memset(dis, 0xcf, sizeof(dis)), dis[S] = 0, q.push(S), in[S] = true;
  while (!q.empty()) {
    int x = q.front();
    q.pop(), in[x] = false;
    for (int i = head[x]; i != -1; i = edge[i].next) {
      if (!edge[i].val) continue;
      if (dis[edge[i].to] < dis[x] + edge[i].cost) {
        dis[edge[i].to] = dis[x] + edge[i].cost, fr[edge[i].to] = x,
        id[edge[i].to] = i;
        if (!in[edge[i].to]) in[edge[i].to] = true, q.push(edge[i].to);
      }
    }
  }
  if (dis[T] == dis[T + 1]) return false;
  int x = T, mn = 0x3f3f3f3f;
  while (x != S) mn = min(mn, edge[id[x]].val), x = fr[x];
  cost += dis[T] * mn, x = T;
  while (x != S) edge[id[x]].val -= mn, edge[id[x] ^ 1].val += mn, x = fr[x];
  return true;
}
}  // namespace MCMF
using namespace MCMF;
int main() {
  scanf("%d%d%s%s", &n, &m, s, t), S = 2 * m, T = 2 * m + 1,
                                   memset(head, -1, sizeof(head));
  for (int i = 0; i < n; i++) g[HASH(s[i])][HASH(t[i])]++;
  for (int i = 0; i < m; i++) ae(S, i, 1, 0), ae(i + m, T, 1, 0);
  for (int i = 0; i < m; i++)
    for (int j = 0; j < m; j++) ae(i, j + m, 1, g[i][j]);
  while (SPFA())
    ;
  printf("%d\n", cost);
  for (int i = 0; i < m; i++)
    for (int j = head[i]; j != -1; j = edge[j].next)
      if (edge[j].to >= m && edge[j].to < 2 * m && !edge[j].val)
        putchar(HASH(edge[j].to - m));
  return 0;
}
