#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
template <typename T>
using Pair = array<T, 2>;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, k;
  while (cin >> n >> k) {
    vector<int> p(n);
    vector<int> q(n);
    for (int i = 0; i < (int)n; i++) {
      cin >> p[i];
      p[i]--;
    }
    for (int i = 0; i < (int)n; i++) {
      cin >> q[i];
      q[i]--;
    }
    vector<int> nq(n, -1);
    vector<int> np(n, -1);
    for (int i = 0; i < (int)n - 1; i++) {
      np[p[i]] = p[i + 1];
      nq[q[i]] = q[i + 1];
    };
    ;
    vector<pair<int, int>> tout(n);
    int t = 1;
    vector<int> used(n);
    string label(n, 'a');
    function<void(int, char, bool)> dfs = [&](int i, char c, bool calct) {
      if (i == -1) {
        return;
      }
      if (used[i]) {
        return;
      }
      used[i] = true;
      dfs(np[i], c, calct);
      dfs(nq[i], c, calct);
      if (calct) {
        tout[i].first = t++;
        tout[i].second = i;
      }
      label[i] = c;
    };
    for (int i = 0; i < n; i++) {
      if (!used[i]) dfs(i, 'a', true);
      ;
    }
    sort(tout.rbegin(), tout.rend());
    used.assign(n, false);
    np.assign(n, -1);
    nq.assign(n, -1);
    for (int i = 0; i < (int)n - 1; i++) {
      np[p[i + 1]] = p[i];
      nq[q[i + 1]] = q[i];
    }
    int c = 0;
    char sym = 'a';
    ;
    ;
    ;
    for (int i = 0; i < n; i++) {
      if (!used[tout[i].second]) {
        dfs(tout[i].second, sym++, false);
        sym = min(sym, 'z');
        c++;
      };
    }
    auto ans = label;
    sort(ans.begin(), ans.end());
    auto apply = [](string arr, vector<int> perm) {
      auto res = arr;
      for (int i = 0; i < arr.size(); i++) {
        res[perm[i]] = arr[i];
      }
      return res;
    };
    if (c < k) {
      cout << "NO" << endl;
      break;
    }
    cout << "YES" << endl;
    for (int i = 0; i < (int)n; i++) {
      cout << label[i];
    }
    cout << endl;
  }
}
