Boost library multiprecision feature in C++ - Can not convert integer constant

Hi,

I was doing some test programming with Boost C++ library and was trying use new feature of multiprecision as given below -

#include <\boost\multiprecision\cpp_int.hpp>
#include <iostream>

using boost::multiprecision::int1024_t;
using namespace std;

int1024_t Factorial(int num);

int main()
{
    int1024_t i = 51090942171709440000;//Factorial(21);
    cout << i << " " << sizeof(i) << endl;
}

int1024_t Factorial(int num)
{
    int1024_t fact = 1;
    for(int i=num; i>1; --i)
    {
        fact *= i;
    }

    return fact;
}

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However, when I run this program it gives me below error -

error: conversion from '<unnamed-signed:128>' to non-scalar type 'boost::multiprecision::int1024_t {aka boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<1024u, 1024u, (boost::multiprecision::cpp_integer_type)1u, (boost::multiprecision::cpp_int_check_type)0u, void> >}' requested

But if instead of assigning value directly to variable i, if I assign function call factorial(21) then it calculates and prints the same big integer value correctly? Why such a behavior?? sizeof operator returns 136 bytes.