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

int main(){
	ll n;
  	cin>>n;
  	ll arr[n];
  	for(int i=0;i<n;i++)
      cin>>arr[i];
  	ll ans=0;
  	for(int i=1;i<n;i++){
      ans+=max(0, arr[i-1]-arr[i]);
      arr[i] = arr[i-1];
    }
  	cout<<ans;
}