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EXAMPLE
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For n=3 the a(3) = 153 valid strings are: 000, 001, 002, 003, 004, 005, 006, 007, 008, 010, 020, 030, 040, 050, 060, 070, 080, 101, 110, 111, 112, 113, 114, 115, 116, 117, 118, 121, 131, 141, 151, 161, 171, 181, 202, 212, 220, 221, 222, 223, 224, 225, 226, 227, 228, 232, 242, 252, 262, 272, 282, 303, 313, 323, 330, 331, 332, 333, 334, 335, 336, 337, 338, 343, 353, 363, 373, 383, 404, 414, 424, 434, 440, 441, 442, 443, 444, 445, 446, 447, 448, 454, 464, 474, 484, 505, 515, 525, 535, 545, 550, 551, 552, 553, 554, 555, 556, 557, 558, 565, 575, 585, 606, 616, 626, 636, 646, 656, 660, 661, 662, 663, 664, 665, 666, 667, 668, 676, 686, 707, 717, 727, 737, 747, 757, 767, 770, 771, 772, 773, 774, 775, 776, 777, 778, 787, 808, 818, 828, 838, 848, 858, 868, 878, 880, 881, 882, 883, 884, 885, 886, 887, 888
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