#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  while (cin >> n >> m) {
    int str[550][550], cnt[550], ans[550];
    memset(str, 0, sizeof(str));
    memset(cnt, 0, sizeof(cnt));
    memset(ans, 0, sizeof(ans));
    for (int i = 0; i < m; i++) {
      int u, v;
      cin >> u >> v;
      str[u][v] = str[v][u] = 1;
      cnt[u]++;
      cnt[v]++;
    }
    for (int i = 1; i <= n; i++)
      if (cnt[i] == n - 1) ans[i] = 1;
    for (int i = 1; i <= n; i++)
      if (ans[i] == 0)
        for (int j = i + 1; j <= n; j++)
          if (str[i][j] == 0) ans[j] = 2;
    bool flag = 1;
    for (int i = 1; i <= n; i++) {
      for (int j = i + 1; j <= n; j++) {
        if ((abs(ans[i] - ans[j]) == 2 && str[i][j]) ||
            (abs(ans[i] - ans[j]) < 2 && !str[i][j])) {
          cout << "No" << endl;
          flag = false;
          break;
        }
      }
      if (!flag) break;
    }
    if (!flag) continue;
    cout << "Yes" << endl;
    for (int i = 1; i <= n; i++) printf("%c", ans[i] + 'a');
    cout << endl;
  }
  return 0;
}
