#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;
int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < (N); ++i) cin >> A[i];
  int ans = 0;
  for (int i = 1; i < N; ++i) {
    if (A[i - 1] <= A[i]) continue;
    ans += A[i - 1] - A[i];
    A[i] = A[i - 1];
  }
  cout << ans << endl;
  return 0;
}
