a better prime number checker

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#include <gmp.h>

int main(int argc, char *argv[]) {
    mpz_t number, num2, divisor, starter; //num2 is converted into a double to act as the counter
    long double counter = 0, final = 2;
    int flag = 0, calcs = 1;
    time_t start, end;
    mpz_init (number);
    mpz_init (num2);
    mpz_init (divisor);
    mpz_init (starter);
    printf("Please enter a number.\n");
    mpz_inp_str (number, stdin, 10);
    flag = mpz_cmp_si(number, 2);
    if (flag < 1) {
        printf("Error: Number must be greater than 2. Please run the program and try again.\n");
        return EXIT_FAILURE;
    }
    flag = 0;
    mpz_sqrt (num2, number);
    counter = mpz_get_d (num2);
    mpz_set_ui (starter, 2);
    gmp_printf("Checking %Zd for primality.\nTesting divisibility with all numbers up to %LF\n\nPlease wait...\n", number, counter);
    start = time(NULL);
    do {
        mpz_mod (divisor, number, starter);
        flag = mpz_cmp_si(divisor, 0);
        if (flag == 0) {
            end = time(NULL);
            gmp_printf("%Zd is divisible by %Zd.\n\nOperation took approximately %ld seconds\n", number, starter, (end - start));
            return EXIT_SUCCESS;
        }
        else {
            final += 1;
            mpz_add_ui (starter, starter, 1);
            if (fmod(final, 10000000) == 0) {
                printf("%d0 million calculations and counting...\n", calcs);
                calcs++;
            }
            continue;
        }
    } while (final < counter);
    end = time(NULL);
    gmp_printf("%Zd is prime.\n\nOperation took approximately %ld seconds\n", number, (end - start));
    mpz_clear (number);
    mpz_clear (num2);
    mpz_clear (divisor);
    mpz_clear (starter);
    return 0;
}


//This program cannot check numbers greater than 8.1 * 10^31 accurately
//Compile on linux with this command: gcc file.c -o file -lgmp -lm -O3