If you recognized the first few Bell numbers, you would, of course, have guessed 877 immediately. Otherwise, you would have done like me and headed off to the fantastic On-Line Encyclopedia of Integer Sequences I've just recently discovered. A quick query for:
Would take you straight to sequence A00110, also known as "Bell or exponential numbers: ways of placing n labeled balls into n indistinguishable boxes. It can also be expressed as this short formula (among others):
a(n) = exp(-1)*sum(k=>0, k^n/k!)
I love this, as I've always loved/hated these types of puzzles. And it has many more usefull uses, as is pointed out by their demo.