#include <bits/stdc++.h>
using namespace std;

signed main(){
  int N; cin >> N;
  vector< int > pos( N + 1 );
  for( int i = 0; i < N; ++i ){
    int A; cin >> A;
    pos[ A ] = i;
  }
  for( int i = N - 1; i >= 1; --i )
    if( pos[ i ] > pos[ i + 1 ] )
      cout << i << endl,
      exit( 0 );
  cout << 0 << endl;
  return 0;
}