#include<bits/stdc++.h>
#define int long long
using namespace std;
int n;
int a[10001];
int last=2;
int ans=0;
signed main(){
	cin>>n;
	for (int i=1;i<=n;i++) cin>>a[i];
	for (int i=3;i<n-1;i++){
		if (a[i]>a[i-1] and a[i]>a[i+1]){
			int p=min(a[i],a[last]);
			for (int j=last+1;j<i;j++){
				ans+=p-a[j];
			}
			last=i;
		}
	}
	if (a[n-1]>a[n-2]){
		for (int j=last+1;j<n-1;j++){
			ans+=min(a[last],a[n-1])-a[j];
		}
	}
	cout<<ans<<endl;
	return 0;
}