How many nonnegative integers can be written in the form $a_6\cdot 4^6+a_5\cdot 4^5+a_4\cdot 4^4+a_3\cdot 4^3+a_2\cdot 4^2+a_1\cdot 4^1+a_0\cdot 4^0$, where $a_i\in -1,0,1,2$ for $0\le i\le 6$?