#include <bits/stdc++.h>
using namespace std;
const int N = 505;
vector<int> adj[N];
bool check[N][N];
int ans[N];
bool vis[N];
bool res = true;
void dfs(int n, int u) {
  ans[n] = u;
  vis[n] = true;
  for (auto v : adj[n]) {
    if (!vis[v]) {
      dfs(v, u);
    } else if (vis[v] == true && u == 3 && ans[v] == 1) {
      res = false;
    }
  }
}
int main() {
  int n, m;
  scanf("%d %d", &n, &m);
  for (int i = 1; i <= m; i++) {
    int u, v;
    scanf("%d %d", &u, &v);
    adj[u].push_back(v);
    adj[v].push_back(u);
    check[u][v] = true;
    check[v][u] = true;
  }
  for (int i = 1; i <= n; i++) {
    if (adj[i].size() == n - 1) {
      ans[i] = 2;
      vis[i] = true;
    }
  }
  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;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      res = false;
      break;
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (i == j) continue;
      if (check[i][j]) {
        if (abs(ans[i] - ans[j]) == 2) {
          res = false;
          break;
        }
      } else {
        if (abs(ans[i] - ans[j]) <= 1) {
          res = false;
          break;
        }
      }
    }
  }
  if (!res) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
    for (int i = 1; i <= n; i++) {
      char c = 'a' + ans[i] - 1;
      cout << c;
    }
  }
  return 0;
}
