Language: C++

#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>
#include<map>
#include<unordered_map>
#include<stack>
#include<string>
#include<algorithm>
#include<functional>
#include<cstring>
#include<utility>
#include<bits/stdc++.h>
#include<math.h>
using namespace std;

/**** Type Define ****/

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P> Q;

/**** Const List   ****/

const ll INF = 1LL << 60;
const ll mod = 1000000007;
const ll dx[4] = { 1, 0, -1, 0 };
const ll dy[4] = { 0, -1, 0, 1 };

/**** General Functions ****/
bool isPrime(ll n) {
	if (n == 1) return false;
	for (int i = 2; i < n; i++) {
		if (i*i > n) {
			break;
		}
		if (n%i == 0 && n != i) {
			return false;
		}
	}
	return true;
}

ll gcd(ll a, ll b) {
	if (b == 0) return a;
	return gcd(b, a % b);
}

ll extgcd(ll a, ll b, ll& x, ll& y) {
	if (b == 0) {
		x = 1, y = 0; return a;
	}
	ll q = a / b, g = extgcd(b, a - q * b, x, y);
	ll z = x - q * y;
	x = y;
	y = z;
	return g;
}

ll invmod(ll a, ll m) { // a^-1 mod m
	ll x, y;
	extgcd(a, m, x, y);
	x %= m;
	if (x < 0) x += m;
	return x;
}

ll nCk(ll n, ll k, ll mod) {
	ll ans = 1;
	for (ll i = n, j = 1; j <= k; i--, j++) ans = (((ans * i) % mod) * invmod(j, mod)) % mod;
	return ans;
}

ll lmin(ll a, ll b) { return a > b ? b : a; };

ll lmax(ll a, ll b) { return a > b ? a : b; };

ll lsum(ll a, ll b) { return a + b; };

/**** Zip ****/

template <typename T>
class Zip {
	vector<T> d;
	bool flag;
public:
	Zip() {
		flag = false;
	}
	void add(T x) {
		d.push_back(x);
		flag = true;
	}
	ll getNum(T x) { // T need to have operator < !!
		if (flag) {
			sort(d.begin(), d.end());
			d.erase(unique(d.begin(), d.end()), d.end());
			flag = false;
		}
		return lower_bound(d.begin(), d.end(), x) - d.begin();
	}
	ll size() {
		if (flag) {
			sort(d.begin(), d.end());
			d.erase(unique(d.begin(), d.end()), d.end());
			flag = false;
		}
		return (ll)d.size();
	}
};

/**** Union Find ****/

class UnionFind {
	vector<ll> par, rank; // par > 0: number, par < 0: -par
public:
	void init(ll n) {
		par.resize(n, 1); rank.resize(n, 0);
	}
	ll getSize(ll x) {
		return par[find(x)];
	}
	ll find(ll x) {
		if (par[x] > 0) return x;
		return -(par[x] = -find(-par[x]));
	}
	void merge(ll x, ll y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		if (rank[x] < rank[y]) {
			par[y] += par[x];
			par[x] = -y;
		}
		else {
			par[x] += par[y];
			par[y] = -x;
			if (rank[x] == rank[y]) rank[x]++;
		}
	}
	bool isSame(ll x, ll y) {
		return find(x) == find(y);
	}
};

/**** Segment Tree ****/

template <typename T>
class SegmentTree {
	ll n;
	vector<T> node;
	function<T(T, T)> fun, fun2;
	bool customChange;
	T outValue, initValue;
public:
	void init(ll num, function<T(T, T)> resultFunction, T init, T out, function<T(T, T)> changeFunction = NULL) {
		// changeFunction: (input, beforevalue) => newvalue
		fun = resultFunction;
		fun2 = changeFunction;
		customChange = changeFunction != NULL;
		n = 1;
		while (n < num) n *= 2;
		node.resize(2 * n - 1, init);
		outValue = out;
		initValue = init;
	}
	void valueChange(ll num, T value) {
		num += n - 1;
		if (customChange) node[num] = fun2(value, node[num]);
		else node[num] = value;
		while (num > 0) num = (num - 1) / 2, node[num] = fun(node[num * 2 + 1], node[num * 2 + 2]);
	}
	T rangeQuery(ll a, ll b, ll l = 0, ll r = -1, ll k = 0) { // [a, b)
		if (r == -1) r = n;
		if (a <= l && r <= b) return node[k];
		if (b <= l || r <= a) return outValue;
		ll mid = (l + r) / 2;
		return fun(rangeQuery(a, b, l, mid, 2 * k + 1), rangeQuery(a, b, mid, r, 2 * k + 2));
	}
};

/**** LIS ****/
ll lis(ll* a, ll n, ll* dp) {
	fill(dp, dp + n, INF);//INFを代入
	for (ll i = 0; i < n; i++) *lower_bound(dp, dp + n, a[i]) = a[i];
	return (ll)(lower_bound(dp, dp + n, INF) - dp);
}
/**** main function ****/
ll n,i=0;

int main() {
	cin >> n;
	while (i*i <= N) {
		i++;
	}
	cout<<(i-1)*(i-1)<<endl;
}




Compiler output:
a.cc: In function 'int main()':
a.cc:193:23: error: 'N' was not declared in this scope
  193 |         while (i*i <= N) {
      |                       ^