If (a_1, a_2..... a_17) satisfya
1 + a_2 + a_3 = 1, a_2 + a_3 + a_4 = 2, a_3 + a_4 + a_5 = 3,
a_{15} + a_{16} + a_{17} = 15, a_{16} + a_{17} + a_{1} = 16, a_{17} + a_{1} + a_{2} = 17, what is the value of a_17?
a_17 = -5
+647 
