#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, i, a, b, k = 0, j;
  cin >> n >> m;
  vector<set<int> > vs(n + 1);
  set<int>::iterator it;
  int ara[n + 1];
  memset(ara, -1, sizeof ara);
  for (i = 0; i < m; i++) {
    cin >> a >> b;
    vs[a].insert(b);
    vs[b].insert(a);
  }
  for (i = 1; i <= n; i++) {
    if (vs[i].size() == n - 1) ara[i] = 1;
  }
  for (i = 1; i <= n; i++) {
    if (ara[i] == -1) {
      ara[i] = 0;
      k = i;
      break;
    }
  }
  if (k != 0) {
    for (it = vs[k].begin(); it != vs[k].end(); it++) {
      if (ara[*it] != 1) ara[*it] = 0;
    }
    for (i = 1; i <= n; i++) {
      if (ara[i] == -1) ara[i] = 2;
    }
    for (i = 1; i <= n; i++) {
      for (j = i + 1; j <= n; j++) {
        if ((ara[i] == ara[j] && vs[i].find(j) == vs[i].end()) ||
            (((ara[i] == 0 && ara[j] == 2) || (ara[i] == 2 && ara[j] == 0)) &&
             vs[i].find(j) != vs[i].end())) {
          cout << "No";
          return 0;
        }
      }
    }
  }
  cout << "Yes\n";
  for (i = 1; i <= n; i++) cout << (char)('a' + ara[i]);
  return 0;
}
