#include <cstdio>
#include <cstring>
#include <cmath>
#include <queue>
#include <vector>
#include <algorithm>

#define MAX_BUFF 1024

using namespace std;

vector<int> doors;
int L,K,N;
double result;

void solve()
{
	sort(doors.begin(),doors.end());
	result = 0;
	double sum;
		sum=doors[0];
		sum+=abs(doors[0]-K);
		sum*=doors[0]/(double)L;
		result+=sum;
		sum=L-doors[N-1];
		sum+=abs(doors[N-1]-K);
		sum*=(L-doors[N-1])/(double)L;
		result+=sum;
	for(int i=0;i<N-1;i++) {
		double l=doors[i];
		double r=doors[i+1];
		for(int j=0;j<300;j++) {
			double m = (l+r)/2;
			if(m-doors[i]+abs(K-doors[i])<doors[i+1]-m+abs(K-doors[i+1]))l=m;
			else r=m;
		}

		sum=l-doors[i];
		sum+=abs(doors[i]-K);
		sum*=(l-doors[i])/(double)L;
		result+=sum;

		sum=doors[i+1]-l;
		sum+=abs(doors[i+1]-K);
		sum*=(doors[i+1]-l)/(double)L;
		result+=sum;
	}
	result/=L;
}

void readInput()
{
	FILE* in = fopen("metro.in", "rt");
	fscanf(in, "%d %d %d", &L,&K,&N);
	for (int i = 0; i < N; i++) {
		int temp;
		fscanf(in, "%d", &temp);
		doors.push_back(temp);
	}
	fclose(in);
}

void writeOutput()
{
	FILE* out = fopen("metro.out", "wt");
	fprintf(out, "%.6f\n", result);
	fclose(out);
}

int main(void)
{
	readInput();
	solve();
	writeOutput();
	return 0;
}
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