f(-1) = -1
f(0) = 12
f(1) = 34
f(2) = 56
f(3) = 78
f(4) = 123
f(5) = 456
f(6) = 789
f(7) = 11
f(8) = 22
f(9) = 33
f(10) = 44
f(11) = 55
f(12) = 66
f(13) = 77
f(14) = 88
f(15) = 99
f(16) = 321
f(17) = 654
f(18) = 987
f(19) = 1001
f(20) = -1