#include <bits/stdc++.h>   using namespace std;   int dirty[3009];   int main() {     int n, m;     cin >> n >> m;     for (typeof(0) i = (0); i < (m); i++) cin >> dirty[i];     sort(dirty, dirty + m);     if (!m)       cout <<  YES n ;     else if ((dirty[0] != 1) && (dirty[m - 1] != n)) {       if (m < 3)         cout <<  YES n ;       else {         bool YES = true;         for (typeof(0) i = (0); i < (m - 2); i++)           if ((dirty[i] == (dirty[i + 1] - 1)) &&               (dirty[i + 1] == (dirty[i + 2] - 1)))             YES = false;         if (YES)           cout <<  YES n ;         else           cout <<  NO n ;       }     } else       cout <<  NO n ;     return 0;   }   