#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const int INF = 1000000001;
const long long int LLINF = 1000000000000000001LL;
int main() {
  ios_base::sync_with_stdio(0);
  int n, m;
  cin >> n >> m;
  bool G[n][n];
  for (int i = (0); i <= (n - 1); ++i)
    for (int j = (0); j <= (n - 1); ++j) G[i][j] = 0;
  for (int i = (1); i <= (m); ++i) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    G[u][v] = G[v][u] = 1;
  }
  if (m == n * (n - 1) / 2) {
    cout << "Yes\n";
    for (int i = (1); i <= (n); ++i) cout << "a";
    return 0;
  }
  vector<int> col(n, -1);
  queue<int> Q;
  int root = -1;
  for (int i = (0); i <= (n - 1); ++i) {
    bool hasnoed = false;
    for (int j = (0); j <= (n - 1); ++j)
      if (i != j && !G[i][j]) {
        hasnoed = true;
      }
    if (hasnoed) {
      root = i;
      break;
    }
  }
  if (root != -1) {
    col[root] = 0;
    Q.push(root);
    while (!Q.empty()) {
      int q = Q.front();
      Q.pop();
      for (int v = (0); v <= (n - 1); ++v)
        if (q != v && col[v] == -1 && !G[q][v]) {
          col[v] = 1 - col[q];
          Q.push(v);
        }
    }
  }
  for (int i = (0); i <= (n - 1); ++i)
    for (int j = (i + 1); j <= (n - 1); ++j) {
      if (!G[i][j] && col[i] == col[j]) {
        cout << "No";
        return 0;
      }
    }
  string cand;
  for (int i = (0); i <= (n - 1); ++i) {
    if (col[i] == 0)
      cand += "a";
    else if (col[i] == 1)
      cand += "c";
    else
      cand += "b";
  }
  for (int i = (0); i <= (n - 1); ++i)
    for (int j = (i + 1); j <= (n - 1); ++j) {
      bool needEdge = (abs((int)cand[i] - (int)cand[j]) <= 1);
      if (needEdge != G[i][j]) {
        cout << "No";
        return 0;
      }
    }
  cout << "Yes\n" << cand;
  return 0;
}
