[Gold Ⅱ] 10775 - 공항

import java.io.BufferedReader;
import java.io.InputStreamReader;
public class BOJ_10775 {
    static int[] parent;
    
    static int find(int x){
        if(parent[x] == x) return x;
        return parent[x] = find(parent[x]);
    }

    static void union(int a, int b){
        a = find(a);
        b = find(b);
        if(a != b) parent[b] = a;
    }
    
    public static void main(String[] args) throws Exception{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int N = Integer.parseInt(br.readLine()); // 게이트의 수
        int M = Integer.parseInt(br.readLine()); // 비행기의 수
        int P = 0, D = 0, f = 0;
        boolean flag = false;

        parent = new int[N+1];
        for(int i = 0; i <= N; i++) parent[i] = i;
        for(int i = 0; i < M; i++){
            P = Integer.parseInt(br.readLine());
            if(flag) continue;
            f = find(P);
            if(f > 0){
                union(f-1, f);
                D++;
            }else flag = true;
        }
        System.out.println(D);
    }
}