dsa-competitive-programming/cses/1083.cpp

//URL - https://cses.fi/problemset/task/1083
//CSES
#include <bits/stdc++.h>
using namespace std;

#ifdef LOCAL
#define dbg(x) cerr << #x << " = " << x << endl
#else
#define dbg(x)
#endif

#define ll long long
#define ld long double
#define ar array
#define all(x) (x).begin(), (x).end()
#define sza(x) (int)(x).size()

const ll INF = 1e18;
const int MOD = 1e9 + 7;

void solve() {
    ll n;
    cin>>n;
    ll sum = (n*(n+1))/2;
    int num;
    while(cin>>num){
        sum-=num;

    }
        cout<<sum;

}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int tc = 1;
    // cin >> tc;
    while(tc--) solve();
}

//XOR approach
/*
void solve() {
    int n;
    cin >> n;

    ll x = 0;

    for(int i = 1; i <= n; i++)
        x ^= i;

    for(int i = 0; i < n-1; i++){
        int a;
        cin >> a;
        x ^= a;
    }

    cout << x << "\n";
}
*/