#include <bits/stdc++.h>
using namespace std;
int ctoi(char c) {
  switch (c) {
    case '0':
      return 0;
    case '1':
      return 1;
    case '2':
      return 2;
    case '3':
      return 3;
    case '4':
      return 4;
    case '5':
      return 5;
    case '6':
      return 6;
    case '7':
      return 7;
    case '8':
      return 8;
    case '9':
      return 9;
    default:
      return 0;
  }
}
int main() {
  int N, humidai, total = 0, max;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++) cin >> A.at(i);
  max = A.at(0);
  for (int i = 1; i < N; i++) {
    humidai = 0;
    if (max > A.at(i)) {
      humidai = max - A.at(i);
    } else {
      max = A.at(i);
    }
    total += humidai;
  }
  cout << total << endl;
}
