wifi.cpp

// Michael Pfeifer
// Version 04, 2018/02/17

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef unsigned long long ull;

const ld  pi   = 4.0*atanl(1.0);
const int iinf = 1e9 + 10;
const ll  inf  = 1e18 + 10;
const int mod  = 1000000007;
const ld  prec = .000001;

#define enld endl
#define endl '\n'
#define pb push_back
#define debug(x) cout<<#x<<" -> "<<x<<'\n'
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define uni(x) (x).erase(unique(all(x)), (x).end())
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define umap unordered_map
#define uset unordered_set

template<class TIn>
using indexed_set = tree<
        TIn, null_type, less<TIn>,
        rb_tree_tag, tree_order_statistics_node_update>;

void fast() {
    ios::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
}

void file() {
    auto a = freopen("a.in",  "r", stdin);
    auto b = freopen("a.out", "w", stdout);
    if(!a || !b) cout << "uh oh" << endl;
}





int calc(vector<int>& v, int w) {
    int cnt = 0;

    int curr = -1;
    for(auto i : v) {
        if(curr < i) {
            cnt++;
            curr = i + w;
        }
    }

    return cnt;
}

void solve() {
    int n, m;
    cin >> n >> m;

    vector<int> v(m);
    for(auto& i : v) {
        cin >> i;
    }
    sort(all(v));

    int lo = 0;
    int hi = 2000000;
    while(hi - lo > 0) {
        int mi = (hi+lo)/2;
        int needed = calc(v, mi);

        if(needed > n) {
            lo = mi + 1;
        }
        else {
            hi = mi;
        }
    }

    cout << fixed;
    cout.precision(1);
    cout << lo / 2.0 << endl;
}

int main() {
    //file();
    fast();

    int cases;
    cin >> cases;
    while(cases--) {
        solve();
    }

    return 0;
}