~cpp
#include <iostream>
#include "BigInteger.h"
using BigMath::BigInteger;
#define MAX_NUM 1000
int i, j, k, input;
BigInteger number[MAX_NUM+1];
void init(int num)
{
number[0]=1;
number[1]=2;
number[2]=5;
for(i=3; i<num+1; i++)
number[i]=2*number[i-1]+number[i-2]+number[i-3];
}
int main()
{
init(1000);
while(cin>>input)
cout<< number[input] <<endl;
return 0;
}