The answer is 5.

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The expression is equivalent to saying:

$a^2 + b^2 = c^2$

The smallest Pythagorean triple with all integers is $(3, 4, 5)$

$(3)^2 + (4)^2 = (5)^2$

After comparison of the equations, this implies the result:

$\boxed{c = 5}$