Let $$P_1(x$$) $$=$$ $$x^2$$ $$+$$ $$a_1x$$ $$+$$ $$b_1$$ and $$P_2(x)$$ $$=$$ $$x^2$$ $$+$$ $$a_2x$$ $$+$$ $$b_2$$ be two quadratic polynomials with integer coefficients. Suppose $$a_1$$ and $$a_2$$ are distinct and there exist distinct integers $$m$$ and $$n$$ such that $$P_1(m)$$ $$=$$ $$P_2(n)$$ and also $$P_2(m)$$ $$=$$ $$P_1(n)$$. We can conclude that $$a_1$$ $$-$$ $$a_2$$ is always :