#include <bits/stdc++.h>
using namespace std;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n >> b) & 1; }
inline void set_bit(int& n, int b) { n |= two(b); }
inline void unset_bit(int& n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
bool edge[555][555];
int cnt[555], ans[555];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    edge[x][y] = edge[y][x] = true;
    cnt[x]++, cnt[y]++;
  }
  memset(ans, -1, sizeof ans);
  for (int i = 1; i <= n; i++) {
    if (ans[i] != -1) continue;
    if (cnt[i] == n - 1)
      ans[i] = 1;
    else {
      ans[i] = 0;
      for (int j = i + 1; j <= n; j++) {
        if (!edge[i][j]) ans[j] = 2;
      }
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (edge[i][j]) {
        if (abs(ans[i] - ans[j]) == 2) {
          cout << "No\n";
          return 0;
        }
      } else {
        if (abs(ans[i] - ans[j]) != 2) {
          cout << "No\n";
          return 0;
        }
      }
    }
  }
  cout << "Yes\n";
  for (int i = 1; i <= n; i++) {
    cout << char(ans[i] + 'a');
  }
  return 0;
}
