#include <stdio.h>
int func(int n) {
    int a = 1;
    int b = 2;
    int c;

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

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

    return b;
}

int main(void) {
    int n = 5;
    printf("a%d = %d\n", n, func(n));
    return 0;
}