#include <stdio.h>
//a{n} = 2a{n-1} + a{n-2}, a{1}=2, a{2}=3(再帰なし版)
 
int main(void) {
    int n = 4;
    int a, b = 3, c = 2; 
    for(int i = 3; i <= n; i++){
        a=b;
        c =a;
        b= 2*a+ c;
    }
	return 0;
}
 
 
				I2luY2x1ZGUgPHN0ZGlvLmg+Ci8vYXtufSA9IDJhe24tMX0gKyBhe24tMn0sIGF7MX09MiwgYXsyfT0zKOWGjeW4sOOBquOBl+eJiCkKCmludCBtYWluKHZvaWQpIHsKICAgIGludCBuID0gNDsKICAgIGludCBhLCBiID0gMywgYyA9IDI7IAogICAgZm9yKGludCBpID0gMzsgaSA8PSBuOyBpKyspewogICAgICAgIGE9YjsKICAgICAgICBjID1hOwogICAgICAgIGI9IDIqYSsgYzsKICAgIH0KICAgIHByaW50Zigi5pWw5YiXYSVk44Gu5YCk44GvJWRcbiIsIG4sIGEpOwoJcmV0dXJuIDA7Cn0KCg==