usleep issue with unistd.h

Question

I want to use miliseconds for my program in c, but there was an error. Basicly im trying to do a halfsecond sleep between between the command.

My code :

#include <stdio.h>
#include <cs50.h>
#include <unistd.h>

int msleep(unsigned int tms) 
{
  return usleep(tms * 1000);
}

int main(void)
{
    int i = get_int("Choose a number.\n");
    for (int a = i; a >= 1; a--)
    {
        printf("%d\n", a);
        sleep(500); 
    }
  {
       printf("blast off!\n");
   } 
    }

My error :

clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    Coding.c  -lcrypt -lcs50 -lm -o Coding
Coding.c:7:10: error: implicit declaration of function 'usleep' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  return usleep(tms * 1000);
         ^
1 error generated.
<builtin>: recipe for target 'Coding' failed
make: *** [Coding] Error 1

Thank you