Language: C++

#include<bits/stdc++.h>
using namespace std;
#define gc getchar_unlocked
#define fo(i,n) for(i=0;i<n;i++)
#define Fo(i,k,n) for(i=k;k<n?i<n:i>n;k<n?i+=1:i-=1)
#define ll long long
#define deb(x) cout << #x << "=" << x << endl
#define deb2(x, y) cout << #x << "=" << x << "," << #y << "=" << y << endl
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define clr(x) memset(x, 0, sizeof(x))
#define sortall(x) sort(all(x))
#define tr(it, a) for(auto it = a.begin(); it != a.end(); it++)
#define PI 3.1415926535897932384626
#define FIO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define GET_BIT(n, i) (((n) & (1LL << ((i)-1))) >> ((i)-1)) // i start from 1
#define SET_BIT(n, i) ((n) | (1LL << ((i)-1)))
#define CLR_BIT(n, i) ((n) & ~(1LL << ((i)-1)))
typedef pair<int, int>  pii;
typedef pair<ll, ll>  pl;
typedef vector<int>   vi;
typedef vector<ll>    vl;
typedef vector<pii>   vpii;
typedef vector<pl>    vpl;
typedef vector<vi>    vvi;
typedef vector<vl>    vvl;
void myInit();
int mpow(int base, int exp);
void ipgraph(int m);
void dfs(int u, int par);
const int MOD = 1000000007;
const int N = 3e5, M = N;
//=========TQR==============
vi g[N];

int main() {
	myInit();
	FIO;
	ll a, b, c, d, sn = 0;
	cin >> a >> b >> c >> d;
	cout << max((a * c, max(a * d, max(b * c, b * d)))) << endl;

	return 0;
}

void myInit() {
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
}

int mpow(int base, int exp) {
	base %= MOD;
	int result = 1;
	while (exp > 0) {
		if (exp & 1) result = ((ll)result * base) % MOD;
		base = ((ll)base * base) % MOD;
		exp >>= 1;
	}
	return result;
}

void ipgraph(int n, int m) {
	int i, u, v;
	while (m--) {
		cin >> u >> v;
		g[u - 1].pb(v - 1);
		g[v - 1].pb(u - 1);
	}
}

void dfs(int u, int par) {
	for (int v : g[u]) {
		if (v == par) continue;
		dfs(v, u);
	}
}

Compiler output:
a.cc: In function 'int main()':
a.cc:44:20: error: no matching function for call to 'max(const long long int&)'
   44 |         cout << max((a * c, max(a * d, max(b * c, b * d)))) << endl;
      |                 ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 1 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   template argument deduction/substitution failed:
a.cc:44:20: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   44 |         cout << max((a * c, max(a * d, max(b * c, b * d)))) << endl;
      |                 ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   candidate expects 2 arguments, 1 provided