#include <bits/stdc++.h>

using namespace std;

void solve() {
    int N;
    cin >> N;
    vector<int> A(N);
    for (int i = 0; i < N; i++)
    {
        cin >> A[i];
    }
    long long a_max = 0, ans = 0;
    for (int i = 0; i < N; i++)
    {
        if (a_max > A[i])
        {
            ans += a_max - A[i];
        }
        a_max = max(a_max, A[i]);
    }
    
    cout << ans << endl;
}

int main() {
    solve();
    return 0;
}