Flatland is a country with a number of cities, some of which have space stations. Cities are numbered consecutively and 
each has a road of 1km length connecting it to the next city. It is not a circular route, so the first city doesn't 
connect with the last city. Determine the maximum distance from any city to it's nearest space station.

For example, there are n=3 cities and m=1 of them has a space station, city 1. They occur consecutively along a route.
City 2 is 2-1=1 unit away and city 3 is 3-1=2 units away. City 1 is 0 units from its nearest space station as one is 
located there. The maximum distance is 2.

Input Format:

The first line consists of two space-separated integers, n and m. 
The second line contains m space-separated integers, the indices of each city having a space-station. These values 
are unordered and unique.

Constraints:
1. 1<=n<=10^5
2. 1<=m<=n
3. There will be at least  city with a space station.
4. No city has more than one space station.

Output Format:
Print an integer denoting the maximum distance that an astronaut in a Flatland city would need to travel to reach the 
nearest space station.

Sample input:
5 2
0 4

Sample output:
2