Inverse Mellin Transform

We all know that the Inverse Mellin Transform is
$$left{mathcal{M}^{-1}varphiright}(x) = f(x)=frac{1}{2 pi i} int_{c-i infty}^{c+i infty} x^{-s} varphi(s), mathrm{d}s.$$
So what is my problem? I want to calculate an integral, which looks like:
$$frac{1}{2 pi i} int_{c-i infty}^{c+i infty} x^{s} varphi(s), mathrm{d}s$$
I want to express this integral as a Inverse Mellin Transformation. My idea was to do a small substitution, so that $s rightarrow -s$. But then the integral has different boundaries:
$$-frac{1}{2 pi i} int_{-c+i infty}^{-c-i infty} x^{-s} varphi(s), mathrm{d}s = frac{1}{2 pi i} int_{-c-i infty}^{-c+i infty} x^{-s} varphi(s), mathrm{d}s$$
This is an Inverse Mellin Transform but $crightarrow-c$. How can I get to a real Inverse Mellin Transform?