An affine manifold is a manifold with a flat affine structure, i.e. a torsion-free. at affine connection. We slightly generalize the result of Hirsch and Thurston that if the holonomy of a closed affine manifold is isomorphic to amenable groups amalgamated or HNN-extended along finite groups, then the Euler characteristic of the manifold is zero confirming an old conjecture of Chern. The technique is from Kim and Lee's work using the combinatorial Gauss-Bonnet theorem and taking the means of the angles by amenability. We show that if an even-dimensional manifold is obtained from a connected sum operation from K(pi,1)s with amenable fundamental groups, then the manifold does not admit an affine structure generalizing a result of Smillie.