C++ :

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

int n,a[100100],x;
int main() {
	cin>>n;
	for(int i = 1;i <= n;i++){
		cin>>x;
		a[x]++;
	}
	
	for(int i = 1;i <= n;i++){
		if(a[i] == 0) cout<<i<<" ";
	}
}

Pascal :

var
a:array[1..100000]of longint;
n,m,i,s,f:longint;
begin
  readln(n);
  for i:=1 to n-2 do
  begin
    read(m);
    a[m]:=1;
  end;
  f:=0;
  for i:=1 to n do
    if a[i]=0 then
    if f=0 then
    begin
      write(i);
      f:=1;
    end
    else write(' ',i);
end.