import java.util.*;

public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  }
} Main public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  } main {
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc long sum = 0l; sum 0l long high = sc.nextLong(); high sc.nextLong() sc.nextLong sc for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    } int i = 1; int i = 1; i 1 i < n i n i++ i++ i {
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    } {
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    } long next = sc.nextLong(); next sc.nextLong() sc.nextLong sc if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      } high > next high next {
      	sum += (high - next);
      } sum += (high - next) sum (high - next) high next {
        high = next;
      } high = next high next System.out.println(sum) System.out.println System.out System System.out sum String[] args args public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  }
} public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  }
} Main
import java.util.*;

public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    long sum = 0l;
    long high = sc.nextLong();
    for(int i = 1; i < n; i++){
      long next = sc.nextLong();
      if(high > next){
      	sum += (high - next);
      } else {
        high = next;
      }
    }
    
    System.out.println(sum);
  }
}
