#include<iostream>
#include<vector>
using namespace std;
int main(){
  int n:
  cin >> n;
  vector<int> a(n);
  for(int i = 0; i < n; i++){
    cin >> a[i];
  }
  int ans = 0;
  for(int i = 0; i < n; i++){
    if(a[i] > a[i+1]){
      ans = ans + a[i] - a[i+1];
      a[i+1] = a[i];
    }
  }
  cout << ans << endl;
}