Карты

#define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <vector>
#include <stdio.h>
#include <cmath>
#include <math.h>
#include <queue>
#include <stack>
#include <climits>
#include <deque>
#include <ctime>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int ui;

#define mh() make_heap()
#define poph() pop_heap()
#define pushh() push_heap()
#define sor(n) n.begin(), n.end()
#define mp make_pair
#define files freopen("input.txt", "rt", stdin); freopen("output.txt", "wt", stdout)

#define formx(a1,b1,c1,a2,b2,c2) ((a1*c2-a2*c1)/(a1*b2-b1*a2))
#define formy(a1,b1,c1,a2,b2,c2) ((c1*b2-c2*b1)/(a1*b2-b1*a2))
#define forma(y1,y2) (y2-y1)
#define formb(x1,x2) (x1-x2)
#define formc(x1,y1,x2,y2) (x1*(y2-y1)-y1*(x2-x1))
#define ras(x1,y1,x2,y2) sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1))

struct cord
{
    double x1, y1, x2, y2;
};
struct limcord
{
    double x, y;
    bool good;
};

bool checkparal(double a1, double a2, double b1, double b2)
{
    return a1 / b1 == a2 / b2;
}

limcord findt(cord c, cord now)
{
    double a1 = forma(now.y1, now.y2);
    double b1 = formb(now.x1, now.x2);
    double c1 = formc(now.x1, now.y1, now.x2, now.y2);
    double a2 = forma(c.y1, c.y2);
    double b2 = formb(c.x1, c.x2);
    double c2 = formc(c.x1, c.y1, c.x2, c.y2);

    if (!checkparal(a1, a2, b1, b2))
    {
        double x = formx(a1, b1, c1, a2, b2, c2);
        double y = formy(a1, b1, c1, a2, b2, c2);
        return{ x,y, true };
    }
    else
    {
        return{ NULL,NULL, false };
    }
}

bool checkT(limcord now, cord ot)
{
    if (ras(ot.x1, ot.y1, ot.x2, ot.y2) == ras(ot.x1, ot.y1, now.x, now.y) + ras(ot.x2, ot.y2, now.x, now.y))
        return true;
    else
        return false;
}

vector <ll> fib;
void aadfib()
{
    fib.push_back(1);
    fib.push_back(1);
    for (int i = 3; i <= 1e7; i++)
    {
        fib.push_back(fib[i - 1] + fib[i - 2]);
    }
}
int main()
{
    files;

    int n, m;
    cin >> n >> m;
    if (n < m)
        cout << "NO";
    else
    {
        vector<int>a(n + 1, 1);
        vector <int> k;
        int prom;
        for (int i = 1; i <= m; i++)
        {
            cin >> prom;
            k.push_back(prom);
        }
        sort(sor(k));
        for (int i = 0; i < m; i++)
        {
            if (k[i] == 0)
                if (!a[k[i] + 1])
                {
                    cout << "NO" << endl;
                    return 0;
                }
                else
                    a[k[i] + 1] = 0;
            else
                if (k[i] == n)
                    if (!a[k[i]])
                    {
                        cout << "NO" << endl;
                        return 0;
                    }
                    else
                        a[k[i]] = 0;
                else
                    if (!a[k[i]])
                        if (a[k[i] + 1])
                            a[k[i] + 1] = 0;
                        else
                        {
                            cout << "NO" << endl;
                            return 0;
                        }
                    else
                        a[k[i]] = 0;
        }
        cout << "YES";
    }
    return 0;
}