#include <bits/stdc++.h>
using namespace std;
const int INF = 2e9 + 7;
void fast() {
  ios::sync_with_stdio(0);
  ios_base::sync_with_stdio(0);
  cout.tie(0);
  cin.tie(0);
}
struct Node {
  int sum;
};
vector<Node> mas;
void build(int v, int l, int r) {
  if (l + 1 == r) {
    mas[v].sum = 0;
    return;
  }
  int m = (l + r) / 2;
  build(2 * v + 1, l, m);
  build(2 * v + 2, m, r);
  mas[v].sum = 0;
  return;
}
void update(int v, int l, int r, int ql, int qr) {
  if (l >= qr || r <= ql) {
    return;
  }
  if (l + 1 == r) {
    mas[v].sum = 1 - mas[v].sum;
    return;
  }
  int m = (l + r) / 2;
  update(2 * v + 1, l, m, ql, qr);
  update(2 * v + 2, m, r, ql, qr);
  mas[v].sum = mas[2 * v + 1].sum | mas[2 * v + 2].sum;
  return;
}
int query(int v, int l, int r, int ql, int qr) { return mas[v].sum; }
signed main() {
  fast();
  int n, k;
  cin >> n >> k;
  vector<int> m1(n);
  vector<int> m2(n);
  vector<int> val;
  vector<int> res(n + 1);
  mas.resize(4 * n);
  build(0, 0, n + 1);
  for (int i = 0; i < n; i++) {
    cin >> m1[i];
  }
  for (int i = 0; i < n; i++) {
    cin >> m2[i];
  }
  for (int i = 0; i < n; i++) {
    int x = m1[i];
    int y = m2[i];
    update(0, 0, n + 1, x, x + 1);
    update(0, 0, n + 1, y, y + 1);
    if (query(0, 0, n + 1, 0, n + 1) == 0) {
      val.push_back(i);
    }
  }
  if (val.size() < k) {
    cout << "NO";
    return 0;
  }
  int cnt = 0;
  int l = 0;
  for (auto el : val) {
    while (l <= el) {
      res[m1[l]] = cnt;
      l++;
    }
    cnt++;
  }
  cout << "YES" << endl;
  for (int i = 1; i <= n; i++) {
    if (res[i] > 25) {
      res[i] = 25;
    }
    cout << char(res[i] + 'a');
  }
  return 0;
}
