#include <bits/stdc++.h>
using namespace std;
map<string, int> mp;
map<string, int>::iterator it;
vector<string> v;
int main() {
  int n, m, h, x, y;
  int a[502][502];
  char str[505];
  string s;
  memset(a, 0, sizeof(a));
  str[n] = '\0';
  cin >> n >> m;
  if (m == 0) {
    if (n <= 2) {
      cout << "Yes" << endl;
      if (n == 1)
        cout << "a" << endl;
      else
        cout << "ac" << endl;
    } else {
      cout << "No" << endl;
    }
    return 0;
  }
  for (int i = 1; i <= n; i++) {
    str[i - 1] = 'a';
    a[i][i] = 1;
  }
  for (int i = 0; i < m; i++) {
    cin >> x >> y;
    a[x][y] = 1;
    a[y][x] = 1;
  }
  for (int i = 1; i <= n; i++) {
    s.clear();
    for (int j = 1; j <= n; j++) {
      if (a[i][j] == 0)
        s += '0';
      else
        s += '1';
    }
    mp[s]++;
  }
  if (mp.size() > 3) {
    cout << "No" << endl;
  } else {
    for (it = mp.begin(); it != mp.end(); it++) {
      v.push_back(it->first);
    }
    if (v.size() == 1) {
      for (int i = 0; i < n; i++) str[i] = 'a';
    } else if (v.size() == 2) {
      for (int i = 1; i <= n; i++) {
        s.clear();
        for (int j = 1; j <= n; j++) {
          if (a[i][j] == 0)
            s += '0';
          else
            s += '1';
        }
        for (h = 0; h < v.size(); h++) {
          if (s.compare(v[h]) == 0) break;
        }
        if (h == 0) {
          str[i - 1] = 'a';
        } else
          str[i - 1] = 'c';
      }
    } else {
      sort(v.begin(), v.end());
      for (int i = 1; i <= n; i++) {
        s.clear();
        for (int j = 1; j <= n; j++) {
          if (a[i][j] == 0)
            s += '0';
          else
            s += '1';
        }
        for (h = 0; h < v.size(); h++) {
          if (s.compare(v[h]) == 0) break;
        }
        if (h == 0) {
          str[i - 1] = 'a';
        } else if (h == 1) {
          str[i - 1] = 'c';
        } else {
          str[i - 1] = 'b';
        }
      }
    }
    cout << "Yes" << endl;
    cout << str << endl;
  }
  return 0;
}
