#include <bits/stdc++.h>
using namespace std;
int n, a[100010];
long long res = 0;
int main() {
  cin >> n;
  for (int i = 0; i < n; ++i) cin >> a[i];
  for (int i = 1; i < n; ++i)
    if (a[i - 1] > a[i]) res += a[i - 1] - a[i];
  cout << res;
  return 0;
}
