#include <bits/stdc++.h>
using namespace std;

int main()
{
  int N;
  cin >> N;
    
  vector<int> height(N);
  for(int i = 0; i < N; i++)
  {
    cin >> height.at(i);
  }
  int sum = 0;
  
  for(int i = 0; i < N-1; i++)
  {
    if(height.at(i) > height.at(i + 1) )
    {
      sum += height.at(i) -height(i+1);
      height.at(i+1) = height.at(i);
    }
  }
  cout << sum;
}