Problem:
Score : 300 points

Problem Statement

There is a circular pond with a perimeter of K meters, and N houses around them.

The i-th house is built at a distance of A_i meters from the northmost point of the pond, measured clockwise around the pond.

When traveling between these houses, you can only go around the pond.

Find the minimum distance that needs to be traveled when you start at one of the houses and visit all the N houses.

Constraints

2 \leq K \leq 10^6

2 \leq N \leq 2 \times 10^5

0 \leq A_1 < ... < A_N < K

All values in input are integers.

Input

Input is given from Standard Input in the following format:

K N
A_1 A_2 ... A_N

Output

Print the minimum distance that needs to be traveled when you start at one of the houses and visit all the N houses.

Sample Input 1

20 3
5 10 15

Sample Output 1

10

If you start at the 1-st house and go to the 2-nd and 3-rd houses in this order, the total distance traveled will be 10.

Sample Input 2

20 3
0 5 15

Sample Output 2

10

If you start at the 2-nd house and go to the 1-st and 3-rd houses in this order, the total distance traveled will be 10.

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

//#define _GLIBCXX_DEBUG
#include "bits/stdc++.h"

using namespace std;

//------------------------------- Type Names -------------------------------//

using i64 = int_fast64_t;

using seika = string;
//{akari : 1D, yukari : 2D, maki : 3D} vector
template <class kizuna>
using akari = vector<kizuna>;
template <class yuzuki>
using yukari = akari<akari<yuzuki>>;
template <class tsurumaki>
using maki = akari<yukari<tsurumaki>>;
//{akane : ascending order, aoi : decending order} priority queue
template <class kotonoha>
using akane = priority_queue<kotonoha, akari<kotonoha>, greater<kotonoha>>;
template <class kotonoha>
using aoi = priority_queue<kotonoha>;

//------------------------------- Libraries ---------------------------------//

//------------------------------- Dubug Functions ---------------------------//

inline void print()
{
    cout << endl;
}
template <typename First, typename... Rest>
void print(const First &first, const Rest &... rest)
{
    cout << first << ' ';
    print(rest...);
}

//------------------------------- Solver ------------------------------------//

void solve()
{
    i64 k, n;
    cin >> k >> n;
    akari<i64> house(n), xs(n);
    for (int i = 0; i < n; i++)
    {
        cin >> house[i];
    }
    for (int i = 0; i < n - 1; i++)
    {
        xs[i] = house[i + 1] - house[i];
    }
    xs.back() = xs[0] + k - xs.back();
    cout << k - *max_element(xs.begin(), xs.end()) << endl;
}

int main()
{
    solve();
    return 0;
}