#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a%dの値は%d\n", n, a);
    return 0;
}