#include <bits/stdc++.h>

#define ll long long

using namespace std;

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