#include
#include<bits/stdc++.h>
using namespace std;
#define ll long long
void findwater(int arr[],int n)
{
int lefta[n];
int righta[n];
int ans;
int left=0;
for (int i = 0; i < n-1; ++i)
{
left=max(arr[i],left);
lefta[i]=left;
}
int right=0;
for (int i = n-1; i>=0; --i)
{
right=max(arr[i],right);
righta[i]=right;
}
for (int i = 0; i < n-1; ++i)
{
ans+=min(lefta[i],righta[i])-(arr[i]);
}
cout<<ans;
}
int main() {
int n,target;
cin>>n;
int a[n];
for(int i=0;i<n;i++)
{
cin>>a[i];
}
findwater(a,n);
return 0;
}