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