#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int A[N];
  for (int i = 0; i < N; i++) {
    cin >> A[i];
  }
  int stepSum = 0;
  int beforeMax = A[0];
  for (int i = 1; i < N; i++) {
    if (A[i] > A[i - 1]) {
      beforeMax = A[i];
    }
    if (A[i] < beforeMax) {
      stepSum += beforeMax - A[i];
    }
  }
  cout << stepSum << endl;
}
