Create and write in N number of textfiles

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C++

My assignment is to create a program that would read a number N from a textfile and create multiplication table up to N. Each multiplication table is to be written into separate a separate file. The program would essentially create N number for textfiles.
For example Table1.txt would contain
1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
1 x 4 = 4
Table2.txt would include
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
and so on

So far my code looks like this and will only print all the multiplication tables into one textfile.

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
	//initializations
	int input, product;
	ifstream inputstream;
	ofstream outputstream;
	inputstream.open("input.txt");
	outputstream.open("output.txt");
	
	//Inputting the number from the text file.
	inputstream >> input;
	
	//Creating the multiplication tables
	for (int counter = 1; counter <= input; counter++)
	{	
		for (int counter2 = 1; counter2 <= input; counter2++)
		{
			product = counter * counter2;
			outputstream << counter << " x " << counter2 << " = " << product << endl; 
		}
		outputstream << "==========================================" << endl << "==========================================" << endl;
	}
	return 0;
}

How should I change my code to get what the assignment is asking for? My intial guess was to put the output.open("output.txt") into the loop but when I did that I was only able to get one text file which had the 1's times table even though the number in my input file was 10.

for (int counter = 1; counter <= input; counter++)
{
    outputstream.open("output.txt");
    for (int counter2 = 1; counter2 <= input; counter2++)
    {
        product = counter * counter2;
        outputstream << counter << " x " << counter2 << " = " << product << endl;
    }
    outputstream << "==========================================" << endl << "==========================================" << endl;
}