#include <stdio.h>

int main(void) {
    int n = 4;
    int a, b = 3, c = 2;  // b = a[n-1], c = a[n-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;
}