#include <bits/stdc++.h>
using namespace std;
int a[55][55];
int conv(char c) {
  if ('A' <= c and c <= 'Z') return 27 + c - 'A';
  return c - 'a' + 1;
}
char enc(int x) {
  if (x <= 26) return 'a' + x - 1;
  return 'A' + x - 27;
}
const int INF = 8000000;
int main() {
  int n, len;
  string from, to;
  cin >> len >> n;
  cin >> from >> to;
  int m = n;
  for (int i = 0; i < len; i++) {
    a[conv(from[i])][conv(to[i])]++;
  }
  for (int i = 0; i < 55; i++)
    for (int j = 0; j < 55; j++) a[i][j] *= -1;
  vector<int> u(n + 1), v(m + 1), p(m + 1), way(m + 1);
  for (int i = 1; i <= n; ++i) {
    p[0] = i;
    int j0 = 0;
    vector<int> minv(m + 1, INF);
    vector<char> used(m + 1, false);
    do {
      used[j0] = true;
      int i0 = p[j0], delta = INF, j1;
      for (int j = 1; j <= m; ++j)
        if (!used[j]) {
          int cur = a[i0][j] - u[i0] - v[j];
          if (cur < minv[j]) minv[j] = cur, way[j] = j0;
          if (minv[j] < delta) delta = minv[j], j1 = j;
        }
      for (int j = 0; j <= m; ++j)
        if (used[j])
          u[p[j]] += delta, v[j] -= delta;
        else
          minv[j] -= delta;
      j0 = j1;
    } while (p[j0] != 0);
    do {
      int j1 = way[j0];
      p[j0] = p[j1];
      j0 = j1;
    } while (j0);
  }
  vector<int> ans(n + 1);
  for (int j = 1; j <= m; ++j) ans[p[j]] = j;
  int val = 0;
  for (int i = 1; i <= n; i++) val += -a[i][ans[i]];
  cout << val << endl;
  for (int i = 1; i <= n; i++) cout << enc(ans[i]);
  return 0;
}
