Missing_Number.cpp

#include <bits/stdc++.h>
#define endl "\n"
#define vll vector<ll>
#define ll long long int
#define mod 1e9 + 7
#define inf 1000000000000000001;

// #define mem(a,val) memset(a,val,sizeof(a))

using namespace std;
ll gcd(ll a, ll b){if (b == 0)return a;return gcd(b, a % b);} //__gcd 
ll lcm(ll a, ll b){return (a/gcd(a,b)*b);}

// Operator overloads
template<typename T> // cin >> vector<T>
istream& operator>>(istream &istream, vector<T> &v){for (auto &it : v)cin >> it;return istream;}
template<typename T> // cout << vector<T>
ostream& operator<<(ostream &ostream, const vector<T> &c) { for (auto &it : c) cout << it << " "; return ostream; }


/////////////////////////////////////////////////////////
// int powerof2(int x){
// 	return (x && !(x & (x-1)));
// }



void solve()
{	
    // https://cses.fi/problemset/result/3550931/
    // ll n, arr_sum = 0;
	// cin >> n;
	// vll arr;
	// for(int i = 0; i < n-1;i++){
	// 	ll it;
	// 	cin >> it;
	// 	arr.push_back(it);
	// 	arr_sum += it;
	// }
	// ll sum = (n * (n+1))/2;
	// cout << sum - arr_sum  << endl

    // https://cses.fi/problemset/result/8981219/
	ll n, temp_sum = 0; cin >> n;
	ll expected_sum = n * (n+1)/2;
	for(ll i = 0;i < n-1; i++){
		ll temp; cin >> temp;
		temp_sum+=temp;
	}
	cout << expected_sum - temp_sum;
	return;
}


int main()
{
	
	#ifndef ONLINE_JUDGE
	freopen("input.txt","r", stdin);
	freopen("output.txt","w", stdout);
	#endif
	std::ios::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);
	// int T;
	// cin>>T;
	// cin.ignore(); // must be there when using getline(cin, s)
	// while(T--){
		solve();
	// }
	return 0;
}