list manipulation – Convolution for large arrays

I am trying to use $texttt{ListConvolve(a, b, {1, -1}, 0)}$ for large arrays $texttt{a}$ and $texttt{b}$ (2048 length) to fast-up the polynomial multiplication. The problem is that I get a wrong answer each time!

I tried the following list enter link description here for $texttt{a}=texttt{b}$, where I should obtain $5.75044*10^-70$ as a first term after the convolution is done, but it didn’t work!