C++ :

#include <bits/stdc++.h>
#define M 100005
using namespace std;

double dis[M], K;
int n;
bool check(double t) {
    double pre = dis[0] + t;
    for (int i = 1; i < n; i++) {
        if (pre + K < dis[i] - t)
            return false;
        pre = min(dis[i] + t, pre + K);
    }
    return true;
}
int main() {
    cin >> K >> n;
    for (int i = 0; i < n; i++)
        scanf("%lf", &dis[i]);

    double L = 0, R = 1e9, ans;
    while (R - L > 1e-6) {
    	double mid = (L + R) / 2;
    	if (check(mid)) ans = mid, R = mid;
    	else L = mid;
	}
    printf("%.3f\n", L);
    return 0;
}