For a natural number n, the symbol n! means n!=1*2*3*...*n, for example: 3! =1*2*3=6, 5! =1*2*3*4*5=120, if 20! =2432902008y7664x000, then how much is xy=.
对于自然数n，符号n!表示n!=1*2*3*…*n，例如：3！=1*2*3=6，5！=1*2*3*4*5=120，如果20！=2432902008y7664x000，那么x-y=多少．
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