#include <iostream>
#include <cstdio>
using namespace std;
int main()
{
    double d = 1000000000000000001;
    cout.setf(ios::fixed);
    cout.precision(0); //0 - число символов после точки
    cout << d << endl;
    printf("%.0lfn",d);
    return 0;
}

1000000000000000000
1000000000000000000

double d1 = 1000000000000000001;
double d2 = 1000000000000000000;

0.1000000000000000055511151231257827021181583404541015625

1 + 1.110223024625156e-16 == 1

1 + 1.110223024625157e-16 != 1