#include <stdio.h>

int func(int n){
    int a = 0, b = 2, c = 1;
    int i;

    if (n == 1) return c;
    if (n == 2) return b;

    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;
}