import java.util.Arrays;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
int[] arr = new int[n];
for (int i = 0; i < n; i++) {
arr[i] = sc.nextInt();
}
sc.close();
int litres = 0;
for (int i = 1; i < n-1; i++) {
int leftMax = Arrays.stream(arr, 0, i).max().getAsInt();
int rightMax = Arrays.stream(arr, i+1, n).max().getAsInt();
int waterHeight = Math.min(leftMax, rightMax) - arr[i];
if (waterHeight > 0) {
litres += waterHeight;
}
}
System.out.println(litres);
}
}