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