Find spectrum of integral operator in L2

Question

Please help me find the spectrum of operator $A$, I do not speak English well. In Russian forums I did not find the answer.
Find spectrum of integral operator in $L_2$($R$): $$ (Ax)(t) = \int\limits_{-\infty}^{+\infty} \dfrac{x(s)ds}{1+(t-s)^2}\ $$ This is Fredholm integral operator (first type) $$K(t,s) = \dfrac{1}{1+(t-s)^2}$$ Eigenvalues ​​are found from the condition $Ax=\lambda x$
The spectrum will consist of $0$ and its own conceptions