#include <stdio.h>
int main(void) {
int n = 4;
int b = 3, c = 2;
int tmp;
for (int i = 3; i <= n; i++) {
tmp = 2 * b + c;
c = b;
b = tmp;
}
printf("数列a%dの値は%d\n", n, b);
return 0;
}