#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;


int main(){
  int N, M, a;
  cin >> N >> M;
  vector<int> A;

  rep(i,M){
    cin >> a;
    A.push_back(a);
  }
  reverse(ALLOF(A));

  int ret = 0;
  rep(i,A.size()){
    if(ret == 0){
      ret = A[i]-1;
    }else if(ret < A[i]){
      ret--;
    }
  }
  
  cout << ret+1 << endl;
  return 0;
}