Need help with using GMP BigNum Library

#include <gmp.h>

int main(void)
{
    // We declare a few BigNum Integer
    mpz_t I, K, M;
    
    // Some BigNum Floats (Not gonna use them)
    // Just demo
    mpf_t U,J,N;
    
    int n;
    
    // Initialize the Integer I as 0
    // set a value for the rest two (K and M)
    
    // We need to initialize ANY BigNum value
    // Its a must
    
    mpz_init(I); // Initialized with 0
    
    mpz_init(K);
    
    mpz_set_si(K, -3000000);
    // si means Signed Integer
    // set_* is used to set values
    
    mpz_init_set_ui(M, 999999999);
    // Initialized and set via Unsigned at a time
    
    // Similarly one can convert via Double, Strings, etc
    // and even set via other mpz_t values
    // Also use xxx_clear() to clear
    
    // Add M, K and store in I
    mpz_add(I,M,K);
    
    // Subtraction, another example
    mpz_sub(M,K,I);
    
    gmp_printf("I = %Zd, K = %Zd, M = %Zd \n", I, K, M);
    
    // Refer the manual for other _functions_ relating to integers
    
    return 0;
}

/*

Note on GMP in general

There are 3 basic types of Numbers in GMP
    Integer - Represented as "Z"
    Float - Represented as "F"
    Rational - Represented as "Q"

There are some more types, such as Limbs and so,
but I don't know much about them.

So in gmp_printf(), the strings obviously are:
%Z<format> for Integer
%F<format> for Float
%Q<format> for Rational

Where <format> can be any C-allowed printf() type such as:
d - Integer
f - Float
x - Hex

gmp_scanf()'s the same

P.s. Heard that stuff for O/P and I/P are easier in C++ but,
I prefer C
*/