#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
    int n, m;
    cin >> n >> m;
    vector <int> a(m);
    bool bol[n + 1][n + 1];
    bool ans = 1;
    for (int i = 0; i < m; i++)
        cin >> a[i];
    sort(a.begin(), a.end());
    for (int i = 0; i <= n; i++)
        for (int j = 0; j <= n; j++)
            bol[i][j] = 0;
    //_____________________________________
    for (int i = 0; i < m; i++){
        if (a[i] - 1 >= 0 && !bol[a[i]][a[i] - 1]){
            bol[a[i]][a[i] - 1] = 1;
            bol[a[i] - 1][a[i]] = 1;
        }
        else
            if (a[i] + 1 <= n && !bol[a[i]][a[i] + 1]){
                bol[a[i]][a[i] + 1] = 1;
                bol[a[i] + 1][a[i]] = 1;
            }
            else{
                ans = 0;
                break;
            }
    }
    //_____________________________________
    if (ans)
        cout << "YES";
    else
        cout << "NO";
    return 0;
}