include <bits/stdc++.h>

using namespace std;

typedef long long ll; typedef long double ld;

void solve() {

ll n, cur;
cin >> n;
ll ans = (n + 1) * n / 2;

for(ll i = 1; i <= n - 1; i++)
{
    cin >> cur;
    ans -= cur;
}
cout << ans;

}

int main() {

solve();
return 0;

}