#include <iostream>
#define int long long
using namespace std;
int main(){
	ios::sync_with_stdio(false);
	int n;
	cin>>n;
	int arr[n];
	for (int i = 0; i < n; ++i){
		cin>>arr[i];
	}
	int max = arr[0];
	int ans = 0;
	for (int i = 1; i < n; ++i){
		if(arr[i]>max){
			max = arr[i];
		}else{
			ans += max-arr[i];
		}
	}
	cout << ans << endl;
}