#include <stdio.h>
// 練習問題G
int func(int n) {
    int a1 = 1; 
    int a2 = 2;
    int next;
    int i;

    if (n == 1) return a1;
    if (n == 2) return a2;

    for (i = 3; i <= n; i++) {
        next = -2 * a2 + 2 * a1; 
        a1 = a2;   
        a2 = next; 
    }

    return a2;
}

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;
}