Wanna do this 29 years before now?

$\prod _{ n=2 }^{ 1986 } \Bigg({ 1-\frac { 1 }{\displaystyle \sum _{ k=0 }^{ n }{ k } } } \Bigg)$

If the product above equals to $\frac a b$ for some relatively coprime positive integers $a,b$ , find $b-a$ .

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