/**
 * Calculates combinations (nCr) - number of ways to choose r items from n items
 * @param n - Total number of items
 * @param r - Number of items to choose
 * @returns {number} Number of combinations, or NaN for invalid arguments
 * @example nCr(5, 2); // 10
 * @description
 * Calculates the number of ways to choose r items from a set of n items,
 * where the order doesn't matter. Uses the formula nCr = nPr / r!
 */
export declare const nCr: (n: number, r: number) => number;