#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll MOD = 1000000007;
using P = pair<int,int>;
#define rep(i, n) for(int i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()

int main(){
    ll n;
    cin >> n;
    if(n==1){
        cout << 0 << endl; return 0;
    }
    rep(i,n){
        ll p; cin >> p;
        if(n==p){
            cout << (n+1)*n/2 - (i+1)*2 << endl;
            return 0;
        }
    }
}