곡선 $\displaystyle x^ {\frac{2} {3}} +y^ {\frac{2} {3}} =1, x \geq 0, y \geq 0$ 의 길이는?

• ①$\displaystyle \frac{2} {3}$
• ②$\displaystyle \frac{3} {2}$
• ③$\displaystyle \frac{4} {3}$
• ④$\displaystyle \frac{7} {6}$