Over a six$-$year period, the returns to the S&P $500$ Index were:$-23\mathrm{.}4\%,26\mathrm{.}4\%,9\mathrm{.}0\%,3\mathrm{.}0\%,13\mathrm{.}6\%,3\mathrm{.}5\%.$Compute the arithmetic average annual return for the S&P $500$ Index for this time period.