$_{{{\left| I-A \right|}^{2}}=\left| {{\left( I-A \right)}^{2}} \right|=\left| {{I}^{2}}-2AI+{{A}^{2}} \right|=\left| I-2A-I \right|=\left| -2A \right|={{\left( -2 \right)}^{2}}\left| A \right|=4\times 1=4\Rightarrow \left| I-A \right|=\pm 2}^{{{A}^{2}}=-I\Rightarrow \left| {{A}^{2}} \right|=\left| -I \right|\Rightarrow \left| {{A}^{2}} \right|={{\left( -1 \right)}^{2}}\left| I \right|=1\xrightarrow{\left| A \right|\rangle 0}\left| A \right|=1}$