Consider the following C program:#include <stdio.h> int counter = 0; int calc (int a, int b) { int c; counter++; if (b==3) return (a*a*a); else { c = calc(a, b/3); return (c*c*c); } } int main () { calc(4, 81); printf ("%d", counter); }The output of this program is ________.

Let us trace the recursive calls to function calc(a, b) starting with calc(4, 81):

1. Call 1:calc(4, 81)

   • counter is incremented to $1$.

   • Since $b = 81 \neq 3$, it executes the else branch and calls calc(4, 81 / 3)$\rightarrow$calc(4, 27).

2. Call 2:calc(4, 27)

   • counter is incremented to $2$.

   • Since $b = 27 \neq 3$, it executes the else branch and calls calc(4, 27 / 3)$\rightarrow$calc(4, 9).

3. Call 3:calc(4, 9)

   • counter is incremented to $3$.

   • Since $b = 9 \neq 3$, it executes the else branch and calls calc(4, 9 / 3)$\rightarrow$calc(4, 3).

4. Call 4:calc(4, 3)

   • counter is incremented to $4$.

   • Since $b = 3 == 3$, it enters the base case and returns $4 \times 4 \times 4 = 64$ without making further recursive calls.

After all recursive calls complete, printf prints the final value of the global variable counter, which is 4.