How to find out the following integral which is used in solution of wave equation?

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I have to find out the value of following integral $$\int_0^t\int_{x-t+s}^{x+t-s} f(y,s) \ dy \ ds$$ where $f(t,x)$ = \begin{cases} 1 \quad if & -1\leq x\leq 1, \ 0\leq t \leq 1 \\ 0 \ \qquad \text{otherwise} \end{cases} I am completely confused how to do this?