import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();

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

    int n = sc.nextInt();

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

    int n = sc.nextInt();

    long max = sc.nextInt();
    long sum = 0;
    for (int i = 1; i < n; i++) {
      long p = sc.nextInt();
      long step;
      if (max > p) {
        step = max - p;
        sum += step;
      } else {
        max = p;
      }
    }
    System.out.println(sum);
  } main {
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();

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

    int n = sc.nextInt();

    long max = sc.nextInt();
    long sum = 0;
    for (int i = 1; i < n; i++) {
      long p = sc.nextInt();
      long step;
      if (max > p) {
        step = max - p;
        sum += step;
      } else {
        max = p;
      }
    }
    System.out.println(sum);
  }
} Main
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();

    long max = sc.nextInt();
    long sum = 0;
    for (int i = 1; i < n; i++) {
      long p = sc.nextInt();
      long step;
      if (max > p) {
        step = max - p;
        sum += step;
      } else {
        max = p;
      }
    }
    System.out.println(sum);
  }
}
