1 - text

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

int main(int argc, char* argv[])
{
    string s[255];
    ifstream in("in.txt"); // Opening a file for reading

    cout<<"Enter number"<<endl;
    string n;
    cin>>n;

    int num = NULL;
    for (int i = 0; i < n.length(); i++){
        num = num*10 + n[i] - 0x30;   //Convert the entered number to a number
    }


    int i = 1;
    while(getline(in,s[i]))// Read the line from the file
        i++;

    in.close(); //Closed the file

    if (num < i)
    s[num] = s[num] + " " + n ; //We add the number after the required line

    ofstream of("in.txt"); //Opened the file for writing

    for ( int j = 1; j < i; j++)
        {
            of<<s[j];   //Writing lines to a file
            of<<endl;
        }
    of.close(); // Close the file


    return 0;
}