The Fibonacci numbers are the numbers in the following integer sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144

int fib(int n)
{
    if (n <= 1)
        return n;
    return fib(n - 1) + fib(n - 2);
}
 
int main()
{
    int n = 9;
    printf("%dth Fibonacci Number: %d", n, fib(n));
    return 0;
}

Uxntal

@on-reset ( -> )
	#0009 fib
	#010e DEO
	BRK

@fib ( num* -- numfib* )
	#0001 GTH2k ?{ POP2 JMP2r }
	( -1 ) SUB2k fib STH2
	( -2 ) INC2 SUB2 fib STH2r
	ADD2 JMP2r

Titan

using std.manip
using std.cond
using std.out

fib == [: [1 - : 1 - fib ⟷ fib +] [] ⍉ 1 > ?]

9 fib .nl