INMO 1989 Problem 1

Prove that the Polynomial $f(x) = x^{4} + 26x^{3} + 56x^{2} + 78x + 1989$ can't be expressed as a product $f(x) = p(x)q(x)$ , where $p(x)$ and $q(x)$ are both polynomial with integral coefficients and with degree at least $1$ .