#include <bits/stdc++.h>
using namespace std;
int N, M;
bool ac[500][500];
bool isa[500], isc[500];
int main() {
  cin >> N >> M;
  for (int i = 0; i < (int)(N); i++)
    for (int j = 0; j < (int)(N); j++)
      if (i != j) ac[i][j] = 1;
  for (int i = 0; i < (int)(M); i++) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    ac[x][y] = ac[y][x] = 0;
  }
  int a = -1, c = -1;
  for (int i = 0; i < (int)(N); i++)
    for (int j = 0; j < (int)(N); j++) {
      if (ac[i][j]) a = i, c = j;
    }
  if (a == -1) {
    puts("Yes");
    cout << string(N, 'a') << endl;
    return 0;
  }
  for (int i = 0; i < (int)(N); i++) {
    if (ac[a][i]) isc[i] = 1;
    if (ac[c][i]) isa[i] = 1;
    if (isa[i] && isc[i]) {
      puts("No");
      return 0;
    }
  }
  for (int i = 0; i < (int)(N); i++)
    for (int j = 0; j < (int)(N); j++)
      if (i != j) {
        bool isac = (isa[i] && isc[j]) ^ (isa[j] && isc[i]);
        if (ac[i][j] != isac) {
          puts("No");
          return 0;
        }
      }
  puts("Yes");
  string s(N, 'b');
  for (int i = 0; i < (int)(N); i++) {
    if (isa[i]) s[i] = 'a';
    if (isc[i]) s[i] = 'c';
  }
  cout << s << endl;
}
