HDU 1568 Fibonacci

#include <stdio.h>
#include <math.h>
int main()
{
const int N=21;
int f
={0,1};
for (int i=2;i<N;i++) f[i]=f[i-1]+f[i-2];
int n;
while (scanf("%d",&n)!=EOF)
{
if (n<N)
{
printf("%d\n",f
);
continue;
}
double e,a;
e=log10(1/sqrt(5))+n*log10((1+sqrt(5))/2);
a=pow(10.0,e-floor(e));
printf("%d\n",(int)(a*1000));
}
}