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

#define rep(i, n) for (int i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()
const int mod = 1000000007;
const long long INF = 1LL << 60;
using ll = long long;

int main()
{
  int N;
  cin >> N;

  int ans=0;
  vector<int> a(N);
  rep(i,N){
    cin >> a[i];
    if(a[i]==N){
      ans=N*(N+1)/2-(i+1);
    }
  }
  
  cout << ans << endl;
}