Problem:
Score : 300 points

Problem Statement

N persons are standing in a row. The height of the i-th person from the front is A_i.

We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:

Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.

Find the minimum total height of the stools needed to meet this goal.

Constraints

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

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;
import java.io.*;
import java.math.*;
	        public final class Main {
	    	static void solve(int[] arr, int n) {
	    	}
	        public static void main(String args[]) throws IOException {
	        	FastReader sc = new FastReader();
	        	int n = sc.nextInt();
	        	long[] arr = sc.readLongArray(n);
	        	long total = 0;
	        	long max = 0;
	        	for(int i = 1;i < n;i++) {
	        		if(arr[i] < arr[i-1]) {
	        			max = arr[i-1];
	        			total += max - arr[i];
	        			arr[i] = max;
	        		}
	        		else  {
	        			max = 0;
	        		}
 	        	}
	        	pn(total);
	       }
	       static void p(Object p){System.out.print(p);}
	       static void pn(){System.out.println();}
	       static void pn(Object o){System.out.println(o);}
	       
	       static class FastReader {
	       static BufferedReader br;
	       StringTokenizer st;
	       public FastReader() {
	       br = new BufferedReader(new InputStreamReader(System.in));
	       }
	            
	       String next(){
	       while (st == null || !st.hasMoreElements()) {
	            try{
	                st = new StringTokenizer(br.readLine());
	            }
	            catch (IOException  e) {
	                e.printStackTrace();
	            }}
	            return st.nextToken();
	            }
	            
	            int nextInt(){ 
	                return Integer.parseInt(next());
	            }
	            
	            long nextLong(){ 
	                return Long.parseLong(next());
	            }
	            
	            static int max(int[] a){
	            int max = Integer.MIN_VALUE;
	            for(int i : a) max = Math.max(max, i);
	            return max;
	            }
	            
	            public int[] readIntArray(int n) {
	            int[] arr = new int[n];
	            for(int i=0; i<n; ++i) arr[i]=nextInt();
	            return arr;
	            }
	            
	            public long[] readLongArray(int n){
	            long[] arr = new long[n];
	            for(int i=0; i<n; ++i) arr[i]=nextLong();
	            return arr;
	            }
	            
	            public int[][] readIntArray(int m, int n){
	            int[][] arr = new int[m][n];
	            for(int i = 0;i<m;i++) 
	            for(int j = 0;j<n;j++) arr[i][j] = nextInt();
	            return arr;
	            }
	            
	            public String[] readStringArray(int n) {
	            String[] arr = new String[n];
	            for(int i=0; i<n; ++i) arr[i]= nextLine();
	            return arr;
	            }
	            
	            double nextDouble() {return Double.parseDouble(next());}
	            
	            String nextLine() {
	            String str = "";
	            try{str = br.readLine();}
	            catch (IOException e) {e.printStackTrace();}
	            return str;}}
	        }