#include <bits/stdc++.h>
using namespace std;
char ans[600];
vector<int> v[600], m;
bool mark[600];
void dfs(int x) {
  mark[x] = true;
  m.push_back(x);
  for (int i = 0; i < v[x].size(); i++) {
    if (!mark[v[x][i]]) dfs(v[x][i]);
  }
}
int main() {
  int n, mm, t = 0, x, y;
  cin >> n >> mm;
  for (int i = 1; i <= mm; i++) {
    cin >> x >> y;
    v[x].push_back(y);
    v[y].push_back(x);
  }
  for (int i = 1; i <= n; i++)
    if (v[i].size() == n - 1) {
      ans[i] = 'b';
      t++;
      mark[i] = true;
    }
  for (int i = 1; i <= n; i++) {
    if (!mark[i]) {
      dfs(i);
      break;
    }
  }
  x = m.size();
  if (t * (n - 1) - (t * (t - 1)) / 2 + (x * (x - 1)) / 2 +
          ((n - t - x) * (n - t - x - 1)) / 2 !=
      mm) {
    cout << "No";
    return 0;
  }
  cout << "Yes" << '\n';
  for (int i = 0; i < m.size(); i++) ans[m[i]] = 'a';
  for (int i = 1; i <= n; i++) {
    if (ans[i] == 'b' || ans[i] == 'a')
      cout << ans[i];
    else
      cout << "c";
  }
}
