The sum of the series \( 2\times 1\times {}^{20}C_{4}-3\times 2\times {}^{20}C_{5}+4\times 3\times {}^{20}C_{6}-5\times 4\times {}^{20}C_{7}+\ldots +18\times 17\times {}^{20}C_{20} \) equal to _________.