#include<iostream>
#include<algorithm>
using namespace std;
int n , m , sum;
int a[100005] ;
int main(){
    cin >> n >> m ;
    for(int i = 1 ; i <= m ; i ++ ){
        cin >> a[i] ;
    }
    sort(a + 1 , a + m + 1 ) ;
	if(a[m] == n * m){
		sum ++ ;
	    for(int i = m - 1 ; i >= 1 ; i -- ){
	        if(a[i + 1] - a[i] == 1){
	            sum ++ ;
	        }else{
	            break;
	        }
	    }
	}
    cout << sum ;
    return 0;
}