CSES – Missing Number

//C++ language

#include<bits/stdc++.h>
#define int long long
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
int n;
vector<int> v;
signed main(){
    fastio;
    cin>>n;
    for(int i=0;i<n-1;i++){
        int x; cin>>x;
        v.push_back(x);
    }
    v.push_back(0);
    sort(v.begin(),v.end());
    for(int i=0;i<=n;i++){
        if(i != v[i]) return cout<<i<<"\n", 0;
    }
    return 0;
}