If $\mathrm{f}:\{1,2,3,\dots ..\}\to \{0,\pm 1,\pm 2,\dots .\}$ is defined by $\mathrm{f}\left(\mathrm{n}\right)=\left\{\begin{array}{l}\mathrm{n}/2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em} \text{ if }\mathrm{n}\text{ is even }\\ -\left(\frac{\mathrm{n}-1}{2}\right)\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\text{ if }\mathrm{n}\text{ is odd }\end{array}\right.$then ${\mathrm{f}}^{-1}(-100)$ is