Definite Integration

Let $f(x) = x + {a \over {{\pi ^2} - 4}}\sin x + {b \over {{\pi ^2} - 4}}\cos x,x \in R$ be a function which satisfies $f(x) = x + \int\limits_0^{\pi /2} {\sin (x + y)f(y)dy}$. then $(a+b)$ is equal to