#include <stdio.h>
//練習問題G
 
int func(int n){
    int a, b=2, c=1;
    int i;
	if(n==1){
		return 1;
	}
	else if(n==2){
		return 2;
	}
	else{
		for(i=3;i<=n;i++){
			a=-2*b+2*c;
			c=b;
			b=a;
		}		
	return a;
	}
}
int main(void) {
	int n = 3;
    printf("数列anについて, n=%dのときの値は%d\n", n
, func
(n
)); 	return 0;
}
 
				I2luY2x1ZGUgPHN0ZGlvLmg+Ci8v57e057+S5ZWP6aGMRwoKaW50IGZ1bmMoaW50IG4pewogICAgaW50IGEsIGI9MiwgYz0xOwogICAgaW50IGk7CglpZihuPT0xKXsKCQlyZXR1cm4gMTsKCX0KCWVsc2UgaWYobj09Mil7CgkJcmV0dXJuIDI7Cgl9CgllbHNlewoJCWZvcihpPTM7aTw9bjtpKyspewoJCQlhPS0yKmIrMipjOwoJCQljPWI7CgkJCWI9YTsKCQl9CQkKCXJldHVybiBhOwoJfQp9CmludCBtYWluKHZvaWQpIHsKCWludCBuID0gMzsKICAgIHByaW50Zigi5pWw5YiXYW7jgavjgaTjgYTjgaYsIG49JWTjga7jgajjgY3jga7lgKTjga8lZFxuIiwgbiwgZnVuYyhuKSk7CglyZXR1cm4gMDsKfQo=