#include <bits/stdc++.h>
using ll = long long;
using ull = unsigned long long;
constexpr int INF = 1073741823;
constexpr int MINF = -1073741823;
constexpr ll LINF = ll(4661686018427387903);
constexpr ll MOD = 1e9 + 7;
constexpr ll mod = 998244353;
constexpr long double eps = 1e-6;
const long double PI = acosl(-1.0L);
using namespace std;
void scans(string& str) {
  char c;
  str = "";
  scanf("%c", &c);
  if (c == '\n') scanf("%c", &c);
  while (c != '\n' && c != -1 && c != ' ') {
    str += c;
    scanf("%c", &c);
  }
}
void scanc(char& str) {
  char c;
  scanf("%c", &c);
  if (c == -1) return;
  while (c == '\n') {
    scanf("%c", &c);
  }
  str = c;
}
double acot(double x) { return PI / 2 - atan(x); }
ll LSB(ll n) { return (n & (-n)); }
template <typename T>
inline T chmin(T& a, const T& b) {
  if (a > b) a = b;
  return a;
}
template <typename T>
inline T chmax(T& a, const T& b) {
  if (a < b) a = b;
  return a;
}
int main() {
  int n;
  scanf("%d", &n);
  ll ans = 0;
  vector<ll> a(n);
  for (int i = 0; i < (n); ++i) scanf("%lld", &a[i]);
  for (int i = 0; i < (n - 1); ++i) {
    ans += max(0ll, a[i] - a[i + 1]);
    a[i + 1] = max(a[i], a[i + 1]);
  }
  printf("%lld\n", ans);
  return 0;
}
