#include <bits/stdc++.h>
using namespace std;
const long long N = 777;
long long n, k, m;
struct edge {
  long long cost;
  long long cap;
  long long rev;
  long long a, b;
};
map<pair<long long, long long>, long long> mp;
long long p[N], d[N], par[N];
bool used[N];
bool use[N];
long long otv[N];
long long s, t;
long long inf = 1e9;
vector<long long> g[N];
vector<edge> e;
void ford() {
  p[0] = 0;
  for (long long i = 1; i <= m; i++) {
    p[i] = inf;
  }
  for (long long ii = 0; ii < 2 * m; ii++) {
    for (long long j = 0; j < e.size(); j++) {
      if (p[e[j].a] < inf && e[j].cap != 0) {
        p[e[j].b] = min(p[e[j].b], p[e[j].a] + e[j].cost);
      }
    }
  }
}
bool dijkstra() {
  for (long long i = 0; i <= m; i++) {
    d[i] = inf;
    used[i] = 0;
  }
  d[0] = 0;
  while (true) {
    long long mn = inf;
    long long v = -1;
    for (long long i = 0; i <= m; i++) {
      if (!used[i] && d[i] < mn) {
        mn = d[i];
        v = i;
      }
    }
    if (v == -1) {
      break;
    }
    used[v] = 1;
    for (long long j = 0; j < g[v].size(); j++) {
      long long id = g[v][j];
      long long i = e[id].b;
      if (used[i]) {
        continue;
      }
      long long w = e[id].cost + p[v] - p[i];
      if (e[id].cap != 0 && d[i] > d[v] + w) {
        d[i] = d[v] + w;
        par[i] = id;
      }
    }
  }
  return d[t] < inf;
}
long long solve() {
  ford();
  long long ans = 0;
  while (dijkstra()) {
    for (long long i = 0; i <= m; i++) {
      p[i] += d[i];
    }
    long long flow = inf;
    long long x = t;
    while (x != s) {
      long long id = par[x];
      flow = min(flow, e[id].cap);
      x = e[id].a;
    }
    x = t;
    while (x != s) {
      long long id = par[x];
      long long rid = e[id].rev;
      e[id].cap -= flow;
      e[rid].cap += flow;
      ans = ans + flow * e[id].cost;
      x = e[id].a;
    }
  }
  return abs(ans);
}
void add(long long a, long long b, long long cost, long long cap) {
  long long cnt = e.size();
  edge p;
  p.a = a, p.b = b, p.cap = cap, p.cost = cost;
  p.rev = cnt + 1;
  e.push_back(p);
  edge q;
  q.a = b, q.b = a, q.cap = 0, q.cost = -cost;
  q.rev = cnt;
  e.push_back(q);
  g[a].push_back(cnt);
  g[b].push_back(cnt + 1);
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s1, s2;
  cin >> n >> k >> s1 >> s2;
  m = 2 * k + 1;
  for (long long i = 0; i < n; i++) {
    long long x = s1[i] - ('a') + 1;
    long long y = s2[i] - ('a') + 1;
    if (x < 1 || x > 26) {
      x = s1[i] - ('A') + 1 + 26;
    }
    if (y < 1 || y > 26) {
      y = s2[i] - ('A') + 1 + 26;
    }
    mp[{x, y}]++;
  }
  for (auto it = mp.begin(); it != mp.end(); it++) {
    long long x = it->first.first;
    long long y = it->first.second;
    y += k;
    long long price = it->second;
    add(x, y, -price, 1);
  }
  s = 0, t = 2 * k + 1;
  for (long long i = 1; i <= k; i++) {
    add(0, i, 0, 1);
  }
  for (long long i = k + 1; i <= k + k; i++) {
    add(i, t, 0, 1);
  }
  for (long long i = 1; i <= k; i++) {
    for (long long j = 1; j <= k; j++) {
      if (mp[{i, j}] == 0) {
        add(i, k + j, 0, 1);
      }
    }
  }
  cout << solve() << "\n";
  for (auto p : e) {
    if (1 <= p.a && p.a <= k && k + 1 <= p.b && p.b <= 2 * k && p.cap == 0 &&
        e[p.rev].cap != 0) {
      otv[p.a] = p.b - k;
      use[p.b - k] = 1;
    }
  }
  for (long long i = 1; i <= k; i++) {
    if (otv[i] == 0) {
      for (long long j = 1; j <= k; j++) {
        if (!use[j]) {
          use[j] = 1;
          otv[i] = j;
          break;
        }
      }
    }
  }
  for (long long i = 1; i <= k; i++) {
    char c;
    if (otv[i] <= 26) {
      c = otv[i] + ('a') - 1;
    } else {
      c = otv[i] - 26 + ('A') - 1;
    }
    cout << c;
  }
  return 0;
}
