hdu 2045 RPG难题

#include <stdio.h>int main() {    int n,i;    __int64 a[51];    a[1] = 3;    a[2] = 6;    a[3] = 6;    for(i = 4;i <= 50;i ++) {        a[i] = a[i-1] + a[i-2]*2;    }       while(scanf("%d",&n)!=EOF) {        printf("%I64d\n",a[n]);    }    return 0;}