#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    ll n, k;
    cin >> n >> k;
    ll a[k] = {};
    rep(i,k) cin >> a[i];

    int flag = 1;
    rep(i,k-1){
        if(a[i+1]-a[i] == 3) flag = 0;
    }

    if(flag) yn;
    return 0;
}