documentclass{article}
usepackage{pgfplots}
pgfplotsset{compat=1.15}

begin{document}

begin{figure}[h]
 begin{tikzpicture}
        begin{axis}
                [xlabel=$x$,ylabel=$y$,
                xtick={100},ytick={100},
                no marks,axis equal,axis lines=middle,
                xmin=-2,xmax=2,ymin=-2,ymax=2,
                enlargelimits={upper=0.1}]
             addplot[color=black, no markers, samples=1001, samples y=0,
              domain=0:pi, variable=t] ( {(2*cos(t r)} , {sin(t r)} );
            end{axis}

        draw (2.98,4.13) node {$1$};
        draw (2.9,2.38) node {$O$};
        draw (5.88,2.42) node {$2$};
        draw (0.3,2.42) node {$-2$};
 end{tikzpicture}
 end{figure}

end{document}