Is there are a more efficient way to do this task ?

I’m an advanced beginner in C++ and I since I do a lot of competitive programming in high school I could really use some tips for faster and more efficient code.

The task is something like this:
As far as I’m aware, I need to create a file (even tho it doesn’t really say in the task) and put 5 strings into it, and I need to output those 5 strings but they need to be decrypted first.
They’re encrypted through a matrix.
So for example a string bcalkcajk has 9 letters so it can be put in a 3x3 matrix.

b c a

l k c

a j k

We decode the text by reading the string column by column , so column “bla” , “ckj” and “ack” , give blackjack.
If the string’s size is not a square of a number , it should read “GRESKA”.

The strings are:

test.in ::::::::::: output

saoreecessinntfi sesnaestocifreni

pmrrj ie2 :::::::: primjer 2

dobar tekst ::::: GRESKA

borj :::::::::::::: broj

Bjsr aoOm ::::::: Broj Osam

The dots are there just to line up the text, I don’t know how to handle these forum texts properly.

The code works, I just want to know if I can improve it in any way and if anybody could explain text files and some tricks, because I just blindly write and read these files, I don’t know how it actually works…
Here’s my code:

#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ofstream write("test.in");
write << "saoreecessinntfi\npmrrj ie2\ndobar tekst\nborj\nBjsr aoOm";
write.close();
string line;
string x[5];
int i=0;
float root;
int size;
int j;
int demolition;
ifstream read("test.in");
if(read.is_open())
{
	while(getline(read,line))
	{
		root=sqrt(line.size());
		size=line.size();
	if(root==floor(root))
	{
		demolition=1;
		j=0;
		for(int k=0;k<size;k++)
		{
			if(j>=size)
			{
				j=demolition;
				demolition++;
			}
			cout << line[j];
			j=j+root;	
		}
	}
	else
	{
		cout << "GRESKA";
	}
	cout << "\n";
	}
	read.close();
}
else
{
	cout << "Ne mogu otvoriti taj fajl.";
}
return 0;
    }
//