Suppose that the index model for stocks A and $B$ is estimated from excess returns with the following results:

$R_A=2\mathrm{.}00\%+0\mathrm{.}40R_M+e_A$

$R_B=-1\mathrm{.}80\%+0\mathrm{.}90R_M+e_B$

${}_M=15\%$;$R-$square $A=($