程序代码：

#include <stdio.h>

long int fac(int inum);

/*the main function*/
int main(void)
{
    int inum, iresult;
    printf("Please input a number, and if you want exit, input -1:\n");
    while (scanf("%d", &inum), inum!=-1)
    {
        iresult=fac(inum);
        printf("%d's factorial is %ld\n", inum, iresult);
    }
    return 0;
}

/*Count the factorial of inum*/
long int fac(int inum)
{
    long int iresult=1;
    while (inum>0)
    {
        iresult*=inum;
        inum--;
    }
    return iresult;
}