若平面向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足： $\left|2\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|\le 3$；则 $\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}$的最小值是.