#include <stdio.h>

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

    if (n == 1) {
        return a;
    }
    if (n == 2) {
        return b;
    }

    for (i = 3; i <= n; i++) {
        c = 2 * a - 2 * b;
        a = b;
        b = c;
    }

    return c;
}

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;
}