#include <bits/stdc++.h>
using namespace std;
void err(istream_iterator<string> it) {
  cout << '\n';
  ;
}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << ", ";
  err(++it, args...);
}
template <typename T>
inline void Int(T &n) {
  n = 0;
  int f = 1;
  register int ch = getchar();
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') f = -1;
  for (; isdigit(ch); ch = getchar()) n = (n << 3) + (n << 1) + ch - '0';
  n = n * f;
}
template <typename T, typename TT>
inline void Int(T &n, TT &m) {
  Int(n);
  Int(m);
}
template <typename T, typename TT, typename TTT>
inline void Int(T &n, TT &m, TTT &l) {
  Int(n, m);
  Int(l);
}
double ind() {
  double x;
  scanf("%lf", &x);
  return x;
}
int set_1(int n, int pos) { return n = (n | (1 << pos)); }
int reset_0(int n, int pos) { return n = n & ~(1 << pos); }
bool check_bit(int n, int pos) { return n = n & (1 << pos); }
int dx[8] = {1, 0, -1, 0, -1, -1, 1, 1};
int dy[8] = {0, 1, 0, -1, -1, 1, -1, 1};
const int N = (int)1e6 + 5;
const int M = (int)1e9 + 7;
const double pi = 2 * acos(0.0);
const double eps = 1e-9;
int main() {
  int n;
  Int(n);
  int x;
  Int(x);
  int la = x, res = 0;
  for (int i = 2; i <= n; ++i) {
    int x;
    Int(x);
    if (x < la) {
      res += (la - x);
    } else {
      la = x;
    }
  }
  printf("%d\n", res);
  return 0;
}
