/*
 * Matheus Oliveira
 * 23/04/2017
 * 2219.cpp
*/

#include <cstdio>
#include <algorithm>

#define MAXN 100100

int main () {

	int number_cases, road_length, number_pokestops, i, pokestop_positions[MAXN], j, maximum_distance, distance;

	scanf("%d", &number_cases);

	for(i=0; i < number_cases; i++) {

		scanf("%d %d", &road_length, &number_pokestops);

		for(j=0; j < number_pokestops; j++) scanf("%d", &pokestop_positions[j]);
		std::sort(pokestop_positions, pokestop_positions+number_pokestops);

		maximum_distance = 0, distance = 0;

		for(j=0; j < number_pokestops; j++) {

			if(pokestop_positions[j] > road_length) break;

			if(pokestop_positions[j] - distance > maximum_distance) maximum_distance = pokestop_positions[j] - distance;
			distance = pokestop_positions[j];
		}

		if(road_length-distance > maximum_distance) maximum_distance = road_length-distance;

		printf("%d\n", maximum_distance);
	}

	return 0;
}