#include<iostream>
#include<cstdio>
using namespace std;
int abs( int n )
{
    return (n < 0 ? -n : n);
}
int main()
{
    freopen( "metro.in", "r", stdin );
    freopen( "metro.out", "w", stdout );
    int k,l,n,a;double mintime=1000000.0;
    scanf("%d%d%d",&l,&k,&n);
    for( int i = 0; i < n; i++ )
    {
         scanf("%d",&a);
         if(a+abs(a-k)<mintime)
              mintime=(a+abs(a-k))/2.0;
    }
    printf("%.6f",mintime);
    return 0;
}