The Cu$2+$ ion selective electrode is placed in a solution A$($Ca$2+)=0\mathrm{.}001$M and the measured potential is $396\mathrm{.}4$ mV$,$ the same solution with A$($Ca$2+)=0\mathrm{.}001$M and A$($Mg$^2+)=0\mathrm{.}001$M and the measured potential is $336\mathrm{.}4.$ What is the value of KPot Ca$2+$ Mg$2+?$