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//
// main.cpp
// Increasing Arrays
//
// Created by Cyril Kevin on 25/05/2021.
//
#include <iostream>
using namespace std;
int main()
{
long long n, d = 0, count = 0;
cin>>n;
long long a[n];
for (int i = 0; i<n; i++)
cin>>a[i];
for (int i = 0; i<n-1; i++) {
d = 0;
if(a[i+1]<a[i])
{
d = a[i] - a[i+1];
a[i+1] = a[i];
count = count + d;
}
}
cout<<count<<endl;
}