#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6 + 5, inf = 1e18, mod = 1e9 + 7;
const long double pi = 3.141592653589793238;
bool check_prime(long long n) {
  long long flag = 0, i;
  for (i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      flag = 1;
      break;
    }
  }
  if (n == 1) {
    return false;
  } else if (flag == 0 || n == 2 || n == 3) {
    return true;
  } else {
    return false;
  }
}
long long fastexpo(long long a, long long b, long long MOD) {
  long long result = 1;
  while (b > 0) {
    if (b % 2 == 1) {
      result = (result * a) % MOD;
    }
    a = (a * a) % MOD;
    b = b / 2;
  }
  return result;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long i, s, n;
  cin >> n;
  long long a[n];
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  s = 0;
  for (i = 0; i < n - 1; i++) {
    if (a[i] > a[i + 1]) {
      s += (a[i] - a[i + 1]);
      a[i + 1] = a[i];
    }
  }
  cout << s << "\n";
  return 0;
}
