C++ Programme

#include <iostream>
using namespace std;
int difference, day, month, year, leapyear;

int main()

{
    cout << "\n Enter the difference in days and press Enter : ";
    cin >> difference;

    cout << "\n Enter a date in the form of DD MM YYYY and press enter: ";
    cin >> day >> month >> year;

    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
    {
        leapyear = 1;
    }
    else
    {
        leapyear = 0;
    }
    
    day = day - difference;
    if ((month == 4) || (month == 6) || (month == 9) || (month == 11))
    {
        if (day > 30)
        {
            //
            day -= 30;
            month++;
        }
        else if (day < 1)
        {
            month--;
            day = day + 31;
        }
    }
    if ((month == 1) || (month == 3) || (month == 5) || (month == 7) || (month == 8) || (month == 10) || (month == 12))
    {
        if (day > 31)
        {

            day = day - 31;
            month++;
            if (month == 13)
            {
                month = 1;
                year++;
            }
        }

        if (day < 1)
        {
            month--;
            if (month == 0)
            {
                day = day + 31;
                month = 12;
                year--;
            }
            if (month == 2)
            {
                if (leapyear == 1)
                {
                    {
                        if (day > 29)
                        {
                            day = day + 29;
                            month--;
                        }

                        else
                        {
                            if (day > 28)
                            {
                                day = day - 28;
                                month++;
                            }
                            else
                            {
                                day = day + 30;
                            }
                        }
                    }
                }
            }
        }
    }
    cout << "\n The final date is: " << day << "/" << month << "/" << year << endl;
}