已知两个单位向量 $\stackrel{\rightharpoonup }{e_1},\stackrel{\rightharpoonup }{e_2}$的夹角为 $\frac{\pi }{3}$，若向量 $\stackrel{\rightharpoonup }{b_1}=\stackrel{\rightharpoonup }{e_1}-2\stackrel{\rightharpoonup }{e_2}$， $\stackrel{\rightharpoonup }{b_2}=3\stackrel{\rightharpoonup }{e_1}+4\stackrel{\rightharpoonup }{e_2}$，则 $\stackrel{\rightharpoonup }{b_1}·\stackrel{\rightharpoonup }{b_2}=$.