#include <bits/stdc++.h>
const long long mod = 1000000007ll;
const double pi = acos(-1.0);
using namespace std;
std::vector<int> v[505];
int mat[505][505];
int vis[505];
void dfs(int cur, int col) {
  vis[cur] = col;
  for (auto it : v[cur]) {
    if (!vis[it]) {
      vis[it] = col;
      dfs(it, col);
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  while (m--) {
    int a, b;
    cin >> a >> b;
    mat[a][b] = 1;
    mat[b][a] = 1;
    v[a].push_back(b);
    v[b].push_back(a);
  }
  for (int i = 1; i <= n; i++)
    if (v[i].size() == n - 1) {
      vis[i] = 2;
    }
  for (int i = 1; i <= n; i++)
    if (!vis[i]) {
      dfs(i, 1);
      break;
    }
  for (int i = 1; i <= n; i++)
    if (!vis[i]) {
      dfs(i, 3);
      break;
    }
  int ispossible = 1;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      ispossible = 0;
      break;
    }
    for (int j = 1; j <= n; j++) {
      if (i == j) continue;
      if (mat[i][j] && abs(vis[i] - vis[j]) > 1) {
        ispossible = 0;
      }
      if (abs(vis[i] - vis[j]) <= 1 && !mat[i][j]) {
        ispossible = 0;
      }
    }
  }
  if (!ispossible)
    cout << "No\n";
  else {
    cout << "Yes\n";
    for (int i = 1; i <= n; i++) {
      cout << (char)('a' + vis[i] - 1);
    }
    cout << "\n";
  }
  return 0;
}
