// metro.cpp : Defines the entry point for the console application.
//

#include<iostream>
#include<fstream>
#include<cmath>

using namespace std;

int main() {
	ifstream in("metro.in");
	ofstream out("metro.out");

	double l,k;
	int n;
	in>>l>>k>>n;
	double min = 2 * l;
	double* a = new double[n];
	for ( int i = 0; i < n; i ++ ) {
		in>>a[i];
		if ( fabs(k - a[i])) < min ) {
			min = fabs(k - a[i]);
		}
	}
	out<<min<<endl;
	return 0;
}

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