Language: C++

///*.......................Shadman Sakib Utshob............................*///

#include <bits/stdc++.h>
using namespace std;

//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;

#define Boost           ios_base::sync_with_stdio(0);cin.tie(0),cout.tie(0)
#define see(x)        cerr<< __LINE__ << ": " << (#x) << " is "<< (x) <<'\n';
#define pb              push_back
#define seecontent(v) cout << (#v) << " is "; for(auto x : v) { cout << x << " ";} cout << "\n\n";
#define inf             1e10
#define mp              make_pair
#define rep(i,n)        for(int i=0;i<n;i++)
#define rep1(i,n)       for(int i=1;i<=n;i++)
#define ll              long long
#define pii             pair<ll,ll >
#define MOD             1000000007
#define fileout         freopen("output.txt","w",stdout)
#define mem(x,i)        memset(x,i,sizeof x)
#define Ones(x)         __builtin_popcount(x);
#define PI              acos(-1.0)
#define ff              first
#define ss              second
#define T               int t;cin>>t;while(t--)
#define ok              cout<<"ok"<<endl;
#define all(x)          (x).begin(),(x).end()
#define ordered_set     tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define nln	            cout<<endl;
#define maxn 300005

int dx[]= {-1,0,1,0}; //up,right,down,left
int dy[]= {0,1,0,-1};

int main()
{
    Boost;
    ll x=1,a,b,c,d,y,z=0;
    int ar[10];
    cin>>a>>b>>c>>d;
    ar[1]=a;
    ar[2]=b;
    ar[3]=c;
    ar[4]=d;
    if(a==0)
        z++;
    if(b==0)
        z++;
    if(c==0)
        z++;
    if(d==0)
        z++;

    if((b>=0 and d>=0)||(b=<0 and d=<0))
    {
        cout<<b*d<<endl;
        return 0;
    }
    else
    {
        if((a>=0 and b>=0)and(c=<0 and d=<0))
        {
            cout<<a*d<<endl;
            return 0;
        }
        if((a=<0 and b=<0)and(c>=0 and d>=0))
        {
            cout<<b*c<<endl;
            return 0;
        }
        if(b=<0)
        {
            if(c=<0)
            {
                cout<<a*c<<endl;
                return 0;
            }
            else
            {
                cout<<b*c<<endl;
                return 0;
            }
        }
        if(b>=0)
        {
            if(a>=0)
            {
                cout<<a*d<<endl;
                return 0;
            }
            else
            {
                cout<<a*c<<endl;
                return 0;
            }
        }
    }
    if(z==4 || z==3)
    {
        cout<<0<<endl;
        return 0;
    }
    if(z==2)
    {
        for(int i=1; i<=4; i++)
        {
            if(ar[i]!=0)
            {
                x*=ar[i];
            }
        }
        if(x<0)
        {
            cout<<0<<endl;
            return 0;
        }
        else
        {
            cout<<x<<endl;
            return 0;
        }
    }
//    if(z==1)
//    {
//        
//    }


    return 0;

}


Compiler output:
a.cc: In function 'int main()':
a.cc:56:28: error: expected primary-expression before '<' token
   56 |     if((b>=0 and d>=0)||(b=<0 and d=<0))
      |                            ^
a.cc:56:37: error: expected primary-expression before '<' token
   56 |     if((b>=0 and d>=0)||(b=<0 and d=<0))
      |                                     ^
a.cc:63:33: error: expected primary-expression before '<' token
   63 |         if((a>=0 and b>=0)and(c=<0 and d=<0))
      |                                 ^
a.cc:63:42: error: expected primary-expression before '<' token
   63 |         if((a>=0 and b>=0)and(c=<0 and d=<0))
      |                                          ^
a.cc:68:15: error: expected primary-expression before '<' token
   68 |         if((a=<0 and b=<0)and(c>=0 and d>=0))
      |               ^
a.cc:68:24: error: expected primary-expression before '<' token
   68 |         if((a=<0 and b=<0)and(c>=0 and d>=0))
      |                        ^
a.cc:73:14: error: expected primary-expression before '<' token
   73 |         if(b=<0)
      |              ^
a.cc:75:18: error: expected primary-expression before '<' token
   75 |             if(c=<0)
      |                  ^