#include <stdio.h>

int main(void) {
    int n = 4;
    int a, b = 3, c = 2;

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

    printf("数列の%dの値は%d\n", n, a);

    return 0;
}