#include <bits/stdc++.h>
long long int MMM = 1000000007;
int pw(long long int a, long long int b) {
  if (b == 0) return 1;
  long long int t = pw(a, b / 2);
  if (b % 2 == 0)
    return (t * t) % 1000000007;
  else
    return ((t * t) % 1000000007 * a) % 1000000007;
}
using namespace std;
vector<int> pm;
void prm(int n) {
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
  for (int p = 2; p <= n; p++)
    if (prime[p]) pm.push_back(p);
}
bool sortbysecdesc(const pair<long long int, long long int> &a,
                   const pair<long long int, long long int> &b) {
  if (a.second == b.second) return a.first < b.first;
  return a.second > b.second;
}
bool cmp(int x, int y) { return x > y; }
long long int gcd(long long int a, long long int b) {
  if (!a) return b;
  return gcd(b % a, a);
}
map<long long int, long long int> pa;
int find(long long int i) {
  if (pa[i] == 0) return i;
  return pa[i] = find(pa[i]);
}
void join(int i, int j) {
  int a = find(i);
  int b = find(j);
  if (b >= a)
    pa[b] = a;
  else
    pa[a] = b;
}
int dx[] = {-1, -1, -1, 0, 1, 1, 1, 0};
int dy[] = {-1, 0, 1, 1, 1, 0, -1, -1};
const int mx = 510;
int n, m;
bool G[mx][mx];
int a[mx], d[mx];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    G[u][v] = G[v][u] = 1;
    d[u]++, d[v]++;
  }
  for (int i = 1; i <= n; i++) {
    G[i][i] = 1;
    d[i]++;
  }
  int x = 0;
  for (int i = 1; i <= n; i++) {
    if (d[i] == n)
      a[i] = 2;
    else
      x = i;
  }
  if (!x) {
    cout << "YES" << endl;
    for (int i = 1; i <= n; i++) cout << "b";
    return 0;
  }
  a[x] = 1;
  for (int i = 1; i <= n; i++) {
    if (!G[x][i])
      a[i] = 3;
    else if (!a[i])
      a[i] = 1;
  }
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= n; j++) {
      if (G[i][j]) {
        if (abs(a[i] - a[j]) > 1) {
          cout << "NO" << endl;
          return 0;
        }
      } else {
        if (abs(a[i] - a[j]) <= 1) {
          cout << "NO" << endl;
          return 0;
        }
      }
    }
  cout << "YES" << endl;
  for (int i = 1; i <= n; i++) {
    char ccc = a[i] - 1 + 'a';
    cout << ccc;
  }
}
