#include <stdio.h>
#include <stdlib.h>

unsigned long long fib(unsigned long long n) {
    unsigned long long r;          // Declare a local variable r
    if (n == 0) {                  // If n is zero, return 0.
        r = 0;
    } else if (n == 1) {           // If n is one, return 1.
        r = 1;
    } else {                       // Else, calculate the number
        r = fib(n-1) + fib(n-2);   //  to return by utilising
    }                              //  recursion.
    return r;
}

int main(int argc, char *argv[]) {
    unsigned long long n, r;       // Define local variables
    n = atoll(argv[1]);            // Convert the first argument
                                   //  to a long long value and
                                   //  load it into n.
    r = fib(n);                    // Set the variable r to the
                                   //  n'th Fibonacci number.
                                   // Print the number found.
    printf("fib(%lld) = %lld\n", n, r);
    return 0;                      // Return 0 to indicate a
                                   //  successful execution.
}