#include <cstdio>
#include <cstring>
#include <ctime>
#include <algorithm>
#include <vector>

using namespace std;

int main(void) {

    int l, k, n;
    double dist, center;
    
    dist = 1 << 30;
    // dist == K???
    
    FILE* in = fopen("metro.in", "rt");
    fscanf(in, "%d %d %d\n", &l, &k, &n);
    for(int ii = 0; ii < n; ++ii) {
        fscanf(in, "%d", &center);
        double cur_dist = center + (k - center);
        if (cur_dist < dist)
            dist = cur_dist;
    }
    
    fclose(in);
    
    
    FILE* out = fopen("metro.out", "wt");
    fprintf(out, "%.3lf", dist);
    fclose(out);
    
	return 0;
}