#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
long l,k,n,min1,a;
int main()
{
    freopen("metro.in","r",stdin);
    freopen("metro.out","w",stdout);
    scanf("%d%d%d",&l,&k,&n);
    min1=2000000001;
    for(int i=1; i<=n; i++)
    {
        scanf("%d",&a);
        min1=min(a,min1);
    }
    if(min1<=k) printf("%.6lf\n",(double)k);
    else printf("%.6lf\n",(double)(2*min1-k));
    return 0;
}
<!-- 0.0513s -->