Missing Number.cpp

#include<bits/stdc++.h>
using namespace std;
#define INF 2147483647
#define INFL 9223372036854775807
#define pii pair<int,int>
#define F first
#define S second
#define mp make_pair
#define pb push_back
#define ll long long
#define ull unsigned long long
#define M 1000000007
#define FASTIO ios_base::sync_with_stdio(false);cin.tie(NULL); cout.tie(NULL);
#define take(x) scanf("%d",&x)
#define DE(x) printf("\ndebug %d\n",x);
#define vout(x) for(int i=0;i<x.size();i++) cout<<x[i]<<" ";
#define pie acos(-1)
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int main(){
    int t = 1;
    //cin>>t;
    while(t--){
        ll n;
        cin>>n;
        int ara[n-1];
        for(int i=0;i<n-1;i++)
            cin>>ara[i];
        sort(ara,ara+n-1);
        for(int i=0;i<n-1;i++){
            if( ara[i]-1!=i ){
                cout<<i+1;
                return 0;
            }
        }
        cout<<n;
    }
}