程序代码：

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

int judge(__int64 a);

int main(void)
{
    __int64 a;
    while (scanf("%lld", &a) != EOF)
    {
        if (judge(a) == 1)
            printf("It is a prime number.\n");
        else
            printf("It is not a prime number.\n");
    }
    system("pause");
    return 0;
}

int judge(__int64 a)
{
    __int64 i, b =  (int)sqrt((float)a);

        for (i = 2; i <= b; i++)
            if (a % i == 0) break;
            return (i == b + 1);   
}