#include <iostream>
using namespace std;

int a[10010];

int main()
{
    int n, mx = 0, cnt = 0;
    cin >> n;
    for (int i = 1; i <= n - 1; i++)
    {
        cin >> a[i];
    }
    for (int i = 2; i < n; i++)
    {
        bool f = true;
        for (int j = i + 1; j < n; j++)
        {
            if (a[j] < a[j - 1])
            {
                cout << cnt;
                return 0;
            }
        }
        if (a[i] > mx)  mx = a[i];
        cnt += mx - a[i];
    }
    cout << cnt;
    return 0;
}