#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void read(T &ret) {
  char c;
  int sgn;
  if (c = getchar(), c == EOF) return;
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  sgn = (c == '-') ? -1 : 1;
  ret = (c == '-') ? 0 : (c - '0');
  while (c = getchar(), c >= '0' && c <= '9') ret = ret * 10 + (c - '0');
  ret *= sgn;
  return;
}
inline void outi(int x) {
  if (x > 9) outi(x / 10);
  putchar(x % 10 + '0');
}
inline void outl(long long x) {
  if (x > 9) outl(x / 10);
  putchar(x % 10 + '0');
}
using namespace std;
int n;
const int N = 2e5 + 7;
int a[N];
int sum = 0;
int main() {
  cin >> n;
  cin >> a[1];
  for (int i = 2; i <= n; i++) {
    cin >> a[i];
    if (a[i] < a[i - 1]) {
      sum += (a[i - 1] - a[i]);
      a[i] = a[i - 1];
    }
  }
  cout << sum << endl;
}
