Introduction_recursion

#include <stdio.h>

int function(int a){
    if (a == 1) return 2;
    else if (a == 2) return 4;
    else if (a == 3) return 7;
    else return 2 * function(a - 1) - function(a - 2) + 4 * function(a - 3);
}
int main(){
    int number;
    scanf("%d", &number);
    int f = function(number);
    printf("%d", f);
}

/* f(x) = f(x-1) + f(x-2) */