#include <iostream>
#include <algorithm>
#include <string>
#include <cstring>
#include <stack>
using namespace std;

#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
#define SQR(X) ((X)*(X))

typedef long long ll;
typedef unsigned long long ull;
typedef long double lb;
/* ここからが本編 */

int main(void)
{
   int i,j;
   int n,m;
   int a[100000];
   int key;
   cin >> n >> m;
   rep(i,m) cin >> a[i];
   key = a[m-1];
   for(i = m-2;i>-1;i--) {
      if(key == 1) key = a[i];
      else if(a[i] >= key) key--;
   }
   cout << key << endl;
   return 0;
}