Counting

There are 9 such 1-digit numbers, 
9×9 = 81 such 2- digit numbers (since the first digit can be 1 to 9 and the second digit any other number from 0 to 9).

Similarly the number of such 3-digit numbers is 9×9×8 = 648,  since the third digit cannot equal to either of the first two. 

For 4-digit numbers beginning 10, there are 8×7 and there are none beginning 11.
Hence , 9 + 81 + 648 + 56 = 794