#include<stdio.h>
#include<algorithm>
using namespace std;

#define MIN -987654321

int N, M;
double cache[301][301];
double p[305] = { 0, };

double calc(int point, int count)
{
    if (count == 1) return 0;
    if (cache[point][count] != 0.0) return cache[point][count];

    double max = MIN;

    for (int i = point-1; i >= count-2; i--)
    {
        double tmp = calc(i,count - 1);
        if (tmp > p[point] - p[i] || tmp==0) tmp = p[point] - p[i];
        if (max < tmp) max = tmp;
    }
    return cache[point][count] = max;
}

int main()
{
    int T;
    scanf("%d", &T);
    while (T--)
    {
        scanf("%d %d", &M, &N);
        for (int i = 0; i < N; i++)
        {
            for (int j =0; j <=M; j++) cache[i][j] = 0.0;
            scanf("%lf", &p[i]);
        }
        printf("%.2lf\n", calc(N - 1, M));
    }
    return 0;
}