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

int main() {
	ll n;
	cin >> n;
	
	ll mx = INT_MIN;
	ll res = 0;
	
	for(int i = 0; i < n; i++) {
		ll x;
		cin >> x;
		
		if(x > mx) {
			mx = x;
		} else {
			res += (mx - x);
		}
	}
	
	cout << res;
	return 0;
}