You are teaching kindergarten! You wrote down the numbers from 1 to , in order, on a whiteboard. When you weren’t paying attention, one of your students erased one of the numbers. Can you tell which number your mischievous student erased?

输入描述:

The first line of input contains a single integer , which is the number of numbers that you wrote down.
The second line of input contains a string of digits, which represents the numbers you wrote down (minus the one that has been erased). There are no spaces in this string. It is guaranteed to contain all of the numbers from 1 to , in order, except for the single number that the student erased.

输出描述:

Output a single integer, which is the number that the tricky student erased.

5
1235

4

10
1234568910

7

15
1234567891012131415

11

备注: