#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>
#include<stdio.h>
#include<queue>
#include <climits>
#include <map>
#include <set>
const int mod = 1e9 + 7;
const int INF = 1 << 20;
using namespace std;
typedef long long ll;
typedef pair<int,int> P;

int main()
{
    int n,m;
    cin >> n >> m;
    int a[m];
    for(int i = 0; i < m; i++){
        cin >> a[i];
    }
    int tmp = 1;
    for(int i = m-1; i >= 0; i--){
        if(a[i] >= tmp) tmp--;
        if(tmp == 0) tmp = a[i];
    }
    cout << tmp << endl;
}