#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <cmath>
#include <iomanip>

using namespace std;

int n,m;
double dist[200];
bool picked[200];

double darpa();
int getUpperIndex(int index);
int getLowerIndex(int index);
int main(int argc, char const *argv[])
{
    int c;
    cin>>c;
    while(c--){
        memset(picked, 0, sizeof(picked));
        cin>>n>>m;
        for(int i=0; i<m; i++){
            cin>>dist[i];
        }
        sort(dist, dist+m);

        double ret=darpa();
        ret=round(ret *100.00) /100.00;
        cout<<fixed << setprecision(2);
        cout<<ret<<endl;
    }
    return 0;
}


double darpa(){

    picked[0]=true;
    picked[m-1]=true;
    double maximum=dist[m-1]-dist[0];
    for(int left=n-2; left>0; left--){
        double val=0;
         int index=-1;
        for(int i=0; i<m ; i++){
            if(picked[i]) continue;
            int lowerIndex=getLowerIndex(i);
            int upperIndex=getUpperIndex(i);
            double cand=min(dist[i]-dist[lowerIndex], dist[upperIndex]-dist[i]);
            if(val <= cand){
                val=cand;
                index=i;
            }
        }
        maximum=min(maximum, val);
        picked[index]=true;
    }

    return maximum;
}

int getLowerIndex(int index){
    for(int i=index-1; i>=0; --i){
        if(picked[i]){
            return i;
        }
    }
}

int getUpperIndex(int index){
    for(int i=index+1; i<m; ++i){
        if(picked[i]){
            return i;
        }
    }

}