Language: Java

package upsolve;

import java.io.*;
import java.util.Arrays;
import java.util.StringTokenizer;

public class Solution {
    public static class Task {
        public void solve(Scanner sc, PrintWriter pw) throws IOException {
//            int T = sc.nextInt();
//            while (T-- > 0) {
            int k = sc.nextInt();
            int n = sc.nextInt();
            int[] a = new int[n];

            for (int i = 0; i < n; i++)
                a[i] = sc.nextInt();

            // 5 10 15
            // 20-15 + 5 , 5 , 5
            // 0 5 15
            // 20-15 + 0 5 10

            int[] diff = new int[n];
            diff[0] = k - a[n - 1] + a[0];
            for (int i = 1; i < n; i++)
                diff[i] = a[i] - a[i - 1];

            Arrays.sort(diff);
            int sum = 0;
            for (int i = 0; i < n - 1; i++)
                sum += diff[i];

            pw.println(sum);
//            }
        }

        static long TIME_START, TIME_END;

        public static void main(String[] args) throws IOException {
            Scanner sc = new Scanner(System.in);
//        Scanner sc = new Scanner(new FileInputStream("nondec.in"));
            PrintWriter pw = new PrintWriter(new BufferedOutputStream(System.out));
//        PrintWriter pw = new PrintWriter(new FileOutputStream("nondec.out"));

            Runtime runtime = Runtime.getRuntime();
            long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();
            TIME_START = System.currentTimeMillis();
            Task t = new Task();
            t.solve(sc, pw);
            TIME_END = System.currentTimeMillis();
            long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();
            pw.close();
            System.err.println("Memory increased: " + (usedMemoryAfter - usedMemoryBefore) / 1000000);
            System.err.println("Time used: " + (TIME_END - TIME_START) + ".");
        }

        static class Scanner {
            StringTokenizer st;
            BufferedReader br;

            public Scanner(InputStream s) {
                br = new BufferedReader(new InputStreamReader(s));
            }

            public Scanner(FileReader s) throws FileNotFoundException {
                br = new BufferedReader(s);
            }

            public String next() throws IOException {
                while (st == null || !st.hasMoreTokens())
                    st = new StringTokenizer(br.readLine());
                return st.nextToken();
            }

            public int nextInt() throws IOException {
                return Integer.parseInt(next());
            }

            public long nextLong() throws IOException {
                return Long.parseLong(next());
            }

            public String nextLine() throws IOException {
                return br.readLine();
            }

            public double nextDouble() throws IOException {
                return Double.parseDouble(next());
            }

            public boolean ready() throws IOException {
                return br.ready();
            }
        }
    }
}


Compiler output:
Main.java:7: error: class Solution is public, should be declared in a file named Solution.java
public class Solution {
       ^
1 error