#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
int main() {
  int N;
  cin >> N;
  vi 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] < A[i - 1]) {
      ans += A[i - 1] - A[i];
      A[i] = A[i - 1];
    }
  }
  cout << ans << endl;
  return 0;
}
