#include<bits/stdc++.h>
using namespace std;
long long a[100005];
int main()
{
	//freopen("water.in","r",stdin);
	//freopen("water.out","w",stdout);
	int n;cin>>n;
	long long ans=0;
	for (int i=1;i<=n;++i) cin>>a[i];
	for (int i=2;i<n;++i)
	{
		if (a[i-1]>a[i])
		{
			int t=i+1;
			while (t<=n&&a[t]<=a[i-1]) ++t;
			if (t>n) continue;
			int x=min(a[t],a[i-1]);
			for (int j=i;j<t;++j)
			{
				ans+=x-a[j];
			}
			i=t;
		}
	}
	cout<<ans;
	return 0;
}