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cia_rana 言語不問：取り違えは何通り？

Created by cia_rana last modified
codeiq.1641.rb
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# 完全順列, 攪乱順列, derangement
p gets.to_i.times.inject([0,1]){|(a,b),i|[b,i*(a+b)]}[1]

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