#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using ull = unsigned long long;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
vector<int> estora(vector<int> num) {
  int sel = 0;
  while (sel < num.size()) {
    int d = num[sel];
    for (int i = 0; i < num.size(); i++) {
      if (num[i] % d == 0 && num[i] != d && d != 1) {
        num.erase(num.begin() + i);
        i--;
      }
    }
    sel++;
  }
  return num;
}
int gcd(int x, int y) {
  int num[3];
  num[0] = (x > y) ? x : y;
  num[1] = (x <= y) ? x : y;
  num[2] = num[0] % num[1];
  while (num[2]) {
    num[0] = num[1];
    num[1] = num[2];
    num[2] = num[0] % num[1];
  }
  return num[1];
}
vector<ll> fac(ll n, vector<ll> num) {
  ll kazu = n;
  for (int i = 2; i <= n; i++) {
    while (1) {
      if (kazu % i != 0) break;
      kazu /= i;
      num[i]++;
    }
  }
  if (kazu == n && n != 1) num[n]++;
  return num;
  int ans = 0;
  if (kazu != 1) ans++;
  for (int i = 2; i <= sqrt(n); i++) {
    for (int j = 1; j <= num[i]; j++) {
      num[i] -= j;
      ans++;
    }
  }
  return num;
}
const int INF = 1001001001;
const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod - a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1) a *= *this;
    return a;
  }
  mint inv() const { return pow(mod - 2); }
  mint& operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x; }
ostream& operator<<(ostream& os, const mint& a) { return os << a.x; }
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  int ans = 0;
  int mae = a[0];
  for (int i = 1; i < n; i++) {
    if (mae > a[i]) {
      ans += mae - a[i];
      a[i] = mae;
      mae = a[i];
    }
    mae = a[i];
  }
  cout << ans;
  return 0;
}
