#include<iostream>
using ll = long long int;
using namespace std;
int main(){
 int n;
 cin>>n;
 ll ans=0;
 ll *arr = new int[ll];
 for(int i=0;i<n;i++){
   cin>>arr[i];
 }
 for(int i=0;i<n-1;i++){
   if(arr[i]>arr[i+1]){
    ll diff = arr[i] - arr[i+1];
    ans +=diff;
    arr[i+1] +=diff;
   }
 }
 cout<<ans<<endl;
 delete []arr;
 return 0;
}
