#include <bits/stdc++.h>
using namespace std;
int dp[700][700];
bool ok(char a, char b) { return abs(a - b) <= 1; }
int main() {
  ios_base::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    dp[a][b] = 1;
    dp[b][a] = 1;
  }
  vector<char> used(n);
  string s;
  s.resize(n, '?');
  for (int i = 0; i < n; i++) {
    for (int j = i + 1; j < n; j++) {
      if (!dp[i][j]) {
        if (s[i] == '?') s[i] = 'a';
        if (s[j] == '?')
          s[j] = (s[i] == 'a') ? 'c' : 'a';
        else {
          if (ok(s[i], s[j])) {
            cout << "No";
            return 0;
          }
        }
      }
    }
  }
  for (int i = 0; i < n; i++)
    if (s[i] == '?') s[i] = 'b';
  for (int i = 0; i < n; i++)
    for (int j = i + 1; j < n; j++)
      if (ok(s[i], s[j]) ^ dp[i][j]) {
        cout << "No";
        return 0;
      }
  cout << "Yes" << endl << s;
  return 0;
}
