10323 - Factorial! You Must be Kidding!!!

#include<stdio.h>

#define UP 6227020800.0
#define LO 10000

void Call(long long n) {
    if(n % 2) printf("Overflow!\n");    //why the odd numbers?
    else printf("Underflow!\n");
}

int main() {
      long long n, sum;
      while(scanf("%lld",&n) == 1) {
       if(n<0) { Call(n); continue; }
       sum = 1;
       for(int i = 1; i<= n; i++) {
               sum *= i;
               if(sum > UP) break;
       }
       if(sum > UP) printf("Overflow!\n");
       else if(sum<=UP && sum>= LO) printf("%lld\n",sum);
       else printf("Underflow!\n");
      }
      return 0;
}