Let $\theta = \displaystyle\sum_{i=1}^n \lambda_i$ and $\Lambda = \frac{1}{n}\theta$.
Using the result that $x$ is integral if and only if $\mathbb Z [x]$ is a finitely generated $\mathbb Z$-module, we conclude that $\theta$ is integral, being a sum of integral elements. (This requires the assumption that $\lambda_i \neq \lambda_j$ for some $i \neq j$). Here and throughout, integral means integral over $\mathbb Z$.
Let $f(X)\in\mathbb Z[X]$ be the monic irreducible polynomial satisfied by $\theta$. Then, $f(n\Lambda) = 0$. So $\Lambda$ satisfies the polynomial $f(nX)$. Since $f(X)$ is irreducible, so must be $f(nX)$. Thus, $f(nX)$ is the minimal polynomial satisfied by $\Lambda$ and its not monic if $n>1$. This contradicts the assumption that $\Lambda$ is integral, unless the degree of $f$ is 1, in which case $\theta=0$ as required.