int ktnguyento5(int n)
{
    int i;
    int m;
    if(n == 2 || n == 3)
        return 1;
    if (n == 1||n % 2 == 0||n % 3 == 0)
        return 0;
    m = (int)sqrt(n);
    for(i=5;i<=m;i=i+6)
        if(n % i == 0 || n % (i+2) == 0)
            return 0;
    return 1;
}