#include <bits/stdc++.h>
using namespace std;
long long color[260000], in[260000];
vector<long long> adj[260000];
bool visited[260000], visited2[260000], done[600][600];
bool ok = true;
long long n, m;
void dfs(long long i, long long num) {
  if (!ok) return;
  if (color[i] != -1) {
    if (color[i] != num) {
      ok = false;
    }
    return;
  }
  color[i] = num;
  for (long long j = 1; j <= n; j++) {
    if (j != i && !done[j][i]) {
      dfs(j, num ^ 1);
    }
  }
}
int main() {
  long long i, x, y, cnt;
  cin >> n >> m;
  for (i = 0; i < m; i++) {
    cin >> x >> y;
    adj[x].push_back(y);
    adj[y].push_back(x);
    in[x]++;
    in[y]++;
    done[x][y] = done[y][x] = 1;
  }
  memset(color, -1, sizeof(color));
  cnt = 0;
  for (i = 1; i <= n; i++) {
    if (in[i] != n - 1 && color[i] == -1) {
      dfs(i, 0);
      if (!ok) {
        cout << "No";
        return 0;
      }
    }
  }
  string s;
  for (i = 1; i <= n; i++) {
    if (color[i] == 0) {
      s += 'a';
    } else if (color[i] == 1) {
      s += 'c';
    } else {
      s += 'b';
    }
  }
  for (i = 0; i < n; i++) {
    for (long long j = i + 1; j < n; j++) {
      if (abs(s[i] - s[j]) <= 1) {
        if (!done[i + 1][j + 1]) {
          cout << "No";
          return 0;
        }
      } else {
        if (done[i + 1][j + 1]) {
          cout << "No";
          return 0;
        }
      }
    }
  }
  cout << "Yes\n";
  cout << s;
  return 0;
}
