When two thin lenses of focal lengths $f_1$   and $f_2$ are placed in contact with each other along their common principal axis, then the two-lens system is regarded as a single lens of focal length f and $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$  If several thin lenses of focal length $f_{1.}{f}_2,\mathrm{.....}{f}_n$ are placed in contact, then the effective focal length of the combination is given by  $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}+\mathrm{.......}\frac{1}{f_n}$.

 And in terms of power, we can write  $P=P_1+P_2+\mathrm{......}+P_n$. The value of focal length and power of a lens must be used with proper sign consideration.