Expectation of a chi-square ratio

Let $X_{1},X_{2},X_{3},X_{4},X_{5}$ be a random sample from $N(0,1)$ distribution and let $W=\dfrac{X_{1}^{2}}{X_{2}^{2}+X_{3}^{2}+X_{4}^{2}+X_{5}^{2}}$. Then $E(W)$ equals