C++:
#include <iostream>
using namespace std;

int combs(int rem, int open = 0) {
        // Base case
        if (rem + open == 0) {
                return 1;
        }

        // Recursive part
        int count = 0;
        if (rem > 0) {
                count += combs(rem-1, open+1);
        }
        if (open > 0) {
                count += combs(rem, open-1);
        }
        return count;
}

int main(int argc, char** argv) {
        cout << combs(13) << endl;
}




Haskell:
combs :: Int -> Int -> Int
combs rem open
    | rem + open == 0     = 1
    | rem > 0 && open > 0 = (combs (rem-1) (open+1)) + (combs rem (open-1))
    | rem > 0             = combs (rem-1) (open+1)
    | open > 0            = combs rem (open-1)

main = do
    putStrLn $ show $ combs 13 0