Bracket covers sum limits using mathclap environment

I have the following code:

\documentclass{article}\usepackage{mathtools}\begin{document}\[S_1 = \frac{1}{2} \left(\sum_{\mathclap{i = n + 1}}^{n + m} a_i t_i \right)\]\end{document}

And it produces this output:

As you can see, the bracket partially covers the lower sum limit.How can I avoid this?