//
//  main.cpp
//  AtCoder
//
//  Created by atsushi on 2019/12/29.
//

#include <iostream>
#include <vector>
#include <set>
#include <string>
#include <utility>
#include <functional>
#include <queue>
#include <cmath>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>      // std::setprecision
#include <bitset>

#define REP(i, N) for(int i=0; i<(N); i++)
#define ALL(N) (N).begin(), (N).end()
#define REC(f, ...) (void)0; const auto f = [&](const auto f, __VA_ARGS__)
using namespace std;

using ll = long long;
template<typename T> class in {
    T value;
public:
    in() { cin >> value; }
    operator const T&() const { return value; }
    operator T&() { return value; }
    in& operator=(T source) { value = source; return *this; }
};
template<> class in<string> : public string {
public:
    in() { cin >> *this; }
};
template<class T> class inseq : public T {
public:
    template<typename... Arg> inseq(Arg... arg) : T(forward<Arg>(arg)...) { for (auto&& i : *this) cin >> i; }
};
template<class T> using invec = inseq<vector<T>>;

auto solve() {
  in<ll> N;
  invec<ll> I(N);

  ll r = 0;
  
  for (int i = N - 1; i >= 1; i--) {
    if (I[N-1] > I[N]) {
      r += I[N-1] - I[N];
    }
  }
  
  return r;
}


template<typename T> void plot(const T& func) { cout << func() << endl; }
template<> void plot<void()>(void(&func)()) { func(); }
template<> void plot<double()>(double(&func)()) { cout << setprecision(9) << func() << endl; }

int main(int argc, const char * argv[]) {
    // insert code here...
    plot(solve);
    return 0;
}