Problem:
Score : 300 points

Problem Statement

N persons are standing in a row. The height of the i-th person from the front is A_i.

We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:

Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.

Find the minimum total height of the stools needed to meet this goal.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rfor(i, m, n) for (int i = (m); i >= (n); --i)
#define unless(c) if (!(c))
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define range_it(a, l, r) (a).begin() + (l), (a).begin() + (r)

using namespace std;
using ll = long long;
using LD = long double;
using VB = vector<bool>;
using VVB = vector<VB>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using VS = vector<string>;
using VD = vector<LD>;
using PII = pair<int, int>;
using VP = vector<PII>;
using PLL = pair<ll, ll>;
using VPL = vector<PLL>;
template <class T> using PQ = priority_queue<T>;
template <class T> using PQS = priority_queue<T, vector<T>, greater<T>>;
constexpr int inf = 1e9;
constexpr long long inf_ll = 1e18, MOD = 1000000007;
constexpr long double PI = M_PI, EPS = 1e-12;

// --- input --- //
#ifdef _WIN32
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#define fwrite_unlocked fwrite
#define fflush_unlocked fflush
#endif
class Input {
	static int gc() {
		return getchar_unlocked();
	}
	template <class T> static void i(T& v) {
		cin >> v;
	}
	static void i(char& v) {
		while (isspace(v = gc()))
			;
	}
	static void i(bool& v) {
		v = in<char>() != '0';
	}
	static void i(string& v) {
		v.clear();
		char c;
		for (i(c); !isspace(c); c = gc()) v += c;
	}
	static void i(int& v) {
		bool neg = false;
		v = 0;
		char c;
		i(c);
		if (c == '-') {
			neg = true;
			c = gc();
		}
		for (; isdigit(c); c = gc()) v = v * 10 + (c - '0');
		if (neg) v = -v;
	}
	static void i(long long& v) {
		bool neg = false;
		v = 0;
		char c;
		i(c);
		if (c == '-') {
			neg = true;
			c = gc();
		}
		for (; isdigit(c); c = gc()) v = v * 10 + (c - '0');
		if (neg) v = -v;
	}
	static void i(double& v) {
		double dp = 1;
		bool neg = false, adp = false;
		v = 0;
		char c;
		i(c);
		if (c == '-') {
			neg = true;
			c = gc();
		}
		for (; isdigit(c) || c == '.'; c = gc()) {
			if (c == '.')
				adp = true;
			else if (adp)
				v += (c - '0') * (dp *= 0.1);
			else
				v = v * 10 + (c - '0');
		}
		if (neg) v = -v;
	}
	static void i(long double& v) {
		long double dp = 1;
		bool neg = false, adp = false;
		v = 0;
		char c;
		i(c);
		if (c == '-') {
			neg = true;
			c = gc();
		}
		for (; isdigit(c) || c == '.'; c = gc()) {
			if (c == '.')
				adp = true;
			else if (adp)
				v += (c - '0') * (dp *= 0.1);
			else
				v = v * 10 + (c - '0');
		}
		if (neg) v = -v;
	}
	template <class T, class U> static void i(pair<T, U>& v) {
		i(v.first);
		i(v.second);
	}
	template <class T> static void i(vector<T>& v) {
		for (auto& e : v) i(e);
	}
	template <size_t N = 0, class T> static void input_tuple(T& v) {
		if constexpr (N < tuple_size_v<T>) {
			i(get<N>(v));
			input_tuple<N + 1>(v);
		}
	}
	template <class... T> static void i(tuple<T...>& v) {
		input_tuple(v);
	}
	struct InputV {
		int n, m;
		InputV(int _n) : n(_n), m(0) {}
		InputV(const pair<int, int>& nm) : n(nm.first), m(nm.second) {}
		template <class T> operator vector<T>() {
			vector<T> v(n);
			i(v);
			return v;
		}
		template <class T> operator vector<vector<T>>() {
			vector<vector<T>> v(n, vector<T>(m));
			i(v);
			return v;
		}
	};

public:
	static string read_line() {
		string v;
		char c;
		for (i(c); c != '\n' && c != '\0'; c = gc()) v += c;
		return v;
	}
	template <class T> static T in() {
		T v;
		i(v);
		return v;
	}
	template <class T> operator T() const {
		return in<T>();
	}
	int operator--(int) const {
		return in<int>() - 1;
	}
	InputV operator[](int n) const {
		return InputV(n);
	}
	InputV operator[](const pair<int, int>& n) const {
		return InputV(n);
	}
	void operator()() const {}
	template <class H, class... T> void operator()(H&& h, T&&... t) const {
		i(h);
		operator()(forward<T>(t)...);
	}

private:
	template <template <class...> class, class...> struct Multiple;
	template <template <class...> class V, class Head, class... Tail> struct Multiple<V, Head, Tail...> {
		template <class... Args> using vec = V<vector<Head>, Args...>;
		using type = typename Multiple<vec, Tail...>::type;
	};
	template <template <class...> class V> struct Multiple<V> { using type = V<>; };
	template <class... T> using multiple_t = typename Multiple<tuple, T...>::type;
	template <size_t N = 0, class T> void in_multiple(T& t) const {
		if constexpr (N < tuple_size_v<T>) {
			auto& vec = get<N>(t);
			using V = typename remove_reference_t<decltype(vec)>::value_type;
			vec.push_back(in<V>());
			in_multiple<N + 1>(t);
		}
	}

public:
	template <class... T> auto multiple(int H) const {
		multiple_t<T...> res;
		while (H--) in_multiple(res);
		return res;
	}
} in;
#define input(T) Input::in<T>()
#define INT input(int)
#define LL input(long long)
#define STR input(string)
#define inputs(T, ...) \
	T __VA_ARGS__;     \
	in(__VA_ARGS__)
#define ini(...) inputs(int, __VA_ARGS__)
#define inl(...) inputs(long long, __VA_ARGS__)
#define ins(...) inputs(string, __VA_ARGS__)

// --- output --- //
struct BoolStr {
	const char *t, *f;
	BoolStr(const char* _t, const char* _f) : t(_t), f(_f) {}
} Yes("Yes", "No"), yes("yes", "no"), YES("YES", "NO"), Int("1", "0");
struct DivStr {
	const char *d, *l;
	DivStr(const char* _d, const char* _l) : d(_d), l(_l) {}
} spc(" ", "\n"), no_spc("", "\n"), end_line("\n", "\n"), comma(",", "\n"), no_endl(" ", "");
class Output {
	BoolStr B{Yes};
	DivStr D{spc};
	void p(int v) const {
		if (v < 0) putchar_unlocked('-'), v = -v;
		char b[10];
		int i = 0;
		while (v) b[i++] = '0' + v % 10, v /= 10;
		if (!i) b[i++] = '0';
		while (i--) putchar_unlocked(b[i]);
	}
	void p(long long v) const {
		if (v < 0) putchar_unlocked('-'), v = -v;
		char b[20];
		int i = 0;
		while (v) b[i++] = '0' + v % 10, v /= 10;
		if (!i) b[i++] = '0';
		while (i--) putchar_unlocked(b[i]);
	}
	void p(bool v) const {
		p(v ? B.t : B.f);
	}
	void p(char v) const {
		putchar_unlocked(v);
	}
	void p(const char* v) const {
		fwrite_unlocked(v, 1, strlen(v), stdout);
	}
	void p(double v) const {
		printf("%.20f", v);
	}
	void p(long double v) const {
		printf("%.20Lf", v);
	}
	template <class T> void p(const T& v) const {
		cout << v;
	}
	template <class T, class U> void p(const pair<T, U>& v) const {
		p(v.first);
		p(D.d);
		p(v.second);
	}
	template <class T> void p(const vector<T>& v) const {
		rep(i, sz(v)) {
			if (i) p(D.d);
			p(v[i]);
		}
	}
	template <class T> void p(const vector<vector<T>>& v) const {
		rep(i, sz(v)) {
			if (i) p(D.l);
			p(v[i]);
		}
	}

public:
	Output& operator()() {
		p(D.l);
		return *this;
	}
	template <class H> Output& operator()(H&& h) {
		p(h);
		p(D.l);
		return *this;
	}
	template <class H, class... T> Output& operator()(H&& h, T&&... t) {
		p(h);
		p(D.d);
		return operator()(forward<T>(t)...);
	}
	template <class It> Output& range(const It& l, const It& r) {
		for (It i = l; i != r; i++) {
			if (i != l) p(D.d);
			p(*i);
		}
		p(D.l);
		return *this;
	}
	template <class T> Output& range(const T& a) {
		range(a.begin(), a.end());
		return *this;
	}
	template <class... T> void exit(T&&... t) {
		operator()(forward<T>(t)...);
		std::exit(EXIT_SUCCESS);
	}
	Output& flush() {
		fflush_unlocked(stdout);
		return *this;
	}
	Output& set(const BoolStr& b) {
		B = b;
		return *this;
	}
	Output& set(const DivStr& d) {
		D = d;
		return *this;
	}
	Output& set(const char* t, const char* f) {
		B = BoolStr(t, f);
		return *this;
	}
} out;

// --- step --- //
template <class T> struct Step {
	class It {
		T a, b, c;

	public:
		constexpr It() : a(T()), b(T()), c(T()) {}
		constexpr It(T _b, T _c, T _s) : a(_b), b(_c), c(_s) {}
		constexpr It& operator++() {
			--b;
			a += c;
			return *this;
		}
		constexpr It operator++(int) {
			It tmp = *this;
			--b;
			a += c;
			return tmp;
		}
		constexpr const T& operator*() const {
			return a;
		}
		constexpr const T* operator->() const {
			return &a;
		}
		constexpr bool operator==(const It& i) const {
			return b == i.b;
		}
		constexpr bool operator!=(const It& i) const {
			return !(b == i.b);
		}
		constexpr T start() const {
			return a;
		}
		constexpr T count() const {
			return b;
		}
		constexpr T step() const {
			return c;
		}
	};
	constexpr Step(T b, T c, T s) : be(b, c, s) {}
	constexpr It begin() const {
		return be;
	}
	constexpr It end() const {
		return en;
	}
	constexpr T start() const {
		return be.start();
	}
	constexpr T count() const {
		return be.count();
	}
	constexpr T step() const {
		return be.step();
	}
	constexpr T sum() const {
		return start() * count() + step() * (count() * (count() - 1) / 2);
	}
	operator vector<T>() const {
		return to_a();
	}
	template <class F> void each(const F& f) const {
		for (T i : *this) f(i);
	}
	auto to_a() const {
		vector<T> res;
		res.reserve(count());
		each([&](T i) { res.push_back(i); });
		return res;
	}
	template <class F, class U = invoke_result_t<F, T>> auto map(const F& f) const {
		vector<U> res;
		res.reserve(count());
		each([&](T i) { res.push_back(f(i)); });
		return res;
	}
	template <class F> auto select(const F& f) const {
		vector<T> res;
		each([&](T i) {
			if (f(i)) res.push_back(i);
		});
		return res;
	}
	template <class F> int count_if(const F& f) const {
		int res = 0;
		each([&](T i) {
			if (f(i)) ++res;
		});
		return res;
	}
	template <class F> optional<T> find_if(const F& f) const {
		for (T i : *this)
			if (f(i)) return i;
		return nullopt;
	}
	template <class F> auto max_by(const F& f) const {
		auto v = map(f);
		return *max_element(v.begin(), v.end());
	}
	template <class F> auto min_by(const F& f) const {
		auto v = map(f);
		return *min_element(v.begin(), v.end());
	}
	template <class F> bool all_of(const F& f) const {
		for (T i : *this)
			if (!f(i)) return false;
		return true;
	}
	template <class F> bool any_of(const F& f) const {
		for (T i : *this)
			if (f(i)) return true;
		return false;
	}
	template <class F, class U = invoke_result_t<F, T>> auto sum(const F& f) const {
		U res = 0;
		each([&](T i) { res += static_cast<U>(f(i)); });
		return res;
	}
	using value_type = T;
	using iterator = It;

private:
	It be, en;
};
template <class T> inline constexpr auto step(T a) {
	return Step<T>(0, a, 1);
}
template <class T> inline constexpr auto step(T a, T b) {
	return Step<T>(a, b - a, 1);
}
template <class T> inline constexpr auto step(T a, T b, T c) {
	return Step<T>(a, a < b ? (b - a - 1) / c + 1 : 0, c);
}

// --- functions --- //
inline namespace {
	template <class T> inline void Sort(T& a) {
		sort(all(a));
	}
	template <class T> inline void RSort(T& a) {
		sort(rall(a));
	}
	template <class T, class F> inline void Sort(T& a, const F& f) {
		sort(all(a), f);
	}
	template <class T, class F> inline void RSort(T& a, const F& f) {
		sort(rall(a), f);
	}
	template <class T> inline T Sorted(T a) {
		Sort(a);
		return a;
	}
	template <class T> inline T RSorted(T a) {
		RSort(a);
		return a;
	}
	template <class T, class F> inline T Sorted(T& a, const F& f) {
		Sort(a, f);
		return a;
	}
	template <class T, class F> inline T RSorted(T& a, const F& f) {
		RSort(a, f);
		return a;
	}
	template <class T, class F> inline void SortBy(T& a, const F& f) {
		sort(all(a), [&](const auto& x, const auto& y) { return f(x) < f(y); });
	}
	template <class T, class F> inline void RSortBy(T& a, const F& f) {
		sort(rall(a), [&](const auto& x, const auto& y) { return f(x) < f(y); });
	}
	template <class T> inline void Reverse(T& a) {
		reverse(all(a));
	}
	template <class T> inline void Unique(T& a) {
		a.erase(unique(all(a)), a.end());
	}
	template <class T> inline void Uniq(T& a) {
		Sort(a);
		Unique(a);
	}
	template <class T> inline void Rotate(T& a, int left) {
		rotate(a.begin(), a.begin() + left, a.end());
	}
	template <class T> inline T Reversed(T a) {
		Reverse(a);
		return a;
	}
	template <class T> inline T Uniqued(T a) {
		Unique(a);
		return a;
	}
	template <class T> inline T Uniqed(T a) {
		Uniq(a);
		return a;
	}
	template <class T> inline T Rotated(T a, int left) {
		Rotate(a, left);
		return a;
	}
	template <class T> inline auto Max(const T& a) {
		return *max_element(all(a));
	}
	template <class T> inline auto Min(const T& a) {
		return *min_element(all(a));
	}
	template <class T> inline int MaxPos(const T& a) {
		return max_element(all(a)) - a.begin();
	}
	template <class T> inline int MinPos(const T& a) {
		return min_element(all(a)) - a.begin();
	}
	template <class T, class F> inline auto MaxBy(const T& a, const F& f) {
		return *max_element(all(a), [&](const auto& x, const auto& y) { return f(x) < f(y); });
	}
	template <class T, class F> inline auto MinBy(const T& a, const F& f) {
		return *min_element(all(a), [&](const auto& x, const auto& y) { return f(x) < f(y); });
	}
	template <class T, class U> inline int Count(const T& a, const U& v) {
		return count(all(a), v);
	}
	template <class T, class F> inline int CountIf(const T& a, const F& f) {
		return count_if(all(a), f);
	}
	template <class T, class U> inline int Find(const T& a, const U& v) {
		return find(all(a), v) - a.begin();
	}
	template <class T, class F> inline int FindIf(const T& a, const F& f) {
		return find_if(all(a), f) - a.begin();
	}
	template <class T, class U = typename T::value_type> inline U Sum(const T& a) {
		return accumulate(all(a), U());
	}
	template <class T, class U> inline bool Includes(const T& a, const U& v) {
		return find(all(a), v) != a.end();
	}
	template <class T, class F> inline auto Sum(const T& v, const F& f) {
		return accumulate(next(v.begin()), v.end(), f(*v.begin()), [&](auto a, auto b) { return a + f(b); });
	}
	template <class T, class U> inline int Lower(const T& a, const U& v) {
		return lower_bound(all(a), v) - a.begin();
	}
	template <class T, class U> inline int Upper(const T& a, const U& v) {
		return upper_bound(all(a), v) - a.begin();
	}
	template <class T, class F> inline void RemoveIf(T& a, const F& f) {
		a.erase(remove_if(all(a), f), a.end());
	}
	template <class F> inline auto Vector(size_t size, const F& f) {
		vector<invoke_result_t<F, size_t>> res(size);
		for (size_t i = 0; i < size; ++i) res[i] = f(i);
		return res;
	}
	template <class T> inline auto Grid(size_t h, size_t w, const T& v = T()) {
		return vector<vector<T>>(h, vector<T>(w, v));
	}
	template <class T> inline auto Slice(const T& v, size_t i, size_t len) {
		return i < v.size() ? T(v.begin() + i, v.begin() + min(i + len, v.size())) : T();
	}
	template <class T, class F> inline auto Each(const T& v, F&& f) {
		for (auto& i : v) f(i);
	}
	template <class T, class F> inline auto Select(const T& v, const F& f) {
		T res;
		for (const auto& e : v)
			if (f(e)) res.push_back(e);
		return res;
	}
	template <class T, class F> inline auto Map(const T& v, F&& f) {
		vector<invoke_result_t<F, typename T::value_type>> res(v.size());
		size_t i = 0;
		for (const auto& e : v) res[i++] = f(e);
		return res;
	}
	template <class T, class F> inline auto MapIndex(const T& v, const F& f) {
		vector<invoke_result_t<F, size_t, typename T::value_type>> res(v.size());
		size_t i = 0;
		for (auto it = v.begin(); it != v.end(); ++it, ++i) res[i] = f(i, *it);
		return res;
	}
	template <class T, class F> inline auto TrueIndex(const T& v, const F& f) {
		vector<size_t> res;
		for (size_t i = 0; i < v.size(); ++i)
			if (f(v[i])) res.push_back(i);
		return res;
	}
	template <class T, class U = typename T::value_type> inline auto Indexed(const T& v) {
		vector<pair<U, int>> res(v.size());
		for (int i = 0; i < (int)v.size(); ++i) res[i] = make_pair(static_cast<U>(v[i]), i);
		return res;
	}
	inline auto operator*(string s, size_t n) {
		string res;
		for (size_t i = 0; i < n; ++i) res += s;
		return res;
	}
	template <class T> inline auto& operator<<(vector<T>& v, const vector<T>& v2) {
		v.insert(v.end(), all(v2));
		return v;
	}
	template <class T> inline T Ceil(T n, T m) {
		return (n + m - 1) / m;
	}
	template <class T> inline T Ceil2(T n, T m) {
		return Ceil(n, m) * m;
	}
	template <class T> inline T Tri(T n) {
		return (n & 1) ? (n + 1) / 2 * n : n / 2 * (n + 1);
	}
	template <class T> inline T nC2(T n) {
		return (n & 1) ? (n - 1) / 2 * n : n / 2 * (n - 1);
	}
	template <class T> inline T Mid(const T& l, const T& r) {
		return l + (r - l) / 2;
	}
	template <class T> inline bool chmax(T& a, const T& b) {
		if (a < b) {
			a = b;
			return true;
		}
		return false;
	}
	template <class T> inline bool chmin(T& a, const T& b) {
		if (a > b) {
			a = b;
			return true;
		}
		return false;
	}
	template <class T> inline bool inRange(const T& v, const T& min, const T& max) {
		return min <= v && v < max;
	}
	template <class T> inline bool isSquere(T n) {
		T s = sqrt(n);
		return s * s == n || (s + 1) * (s + 1) == n;
	}
	template <class T = long long> inline T BIT(int b) {
		return T(1) << b;
	}
	template <class T, class U = typename T::value_type> inline U Gcdv(const T& v) {
		return accumulate(next(v.begin()), v.end(), U(*v.begin()), gcd<U, U>);
	}
	template <class T, class U = typename T::value_type> inline U Lcmv(const T& v) {
		return accumulate(next(v.begin()), v.end(), U(*v.begin()), lcm<U, U>);
	}
	template <class T> inline T Pow(T a, T n) {
		T r = 1;
		while (n > 0) {
			if (n & 1) r *= a;
			a *= a;
			n /= 2;
		}
		return r;
	}
	template <class T> inline T Powmod(T a, T n, T m = MOD) {
		T r = 1;
		while (n > 0) {
			if (n & 1)
				r = r * a % m, n--;
			else
				a = a * a % m, n /= 2;
		}
		return r;
	}
}  // namespace

// --- dump --- //
#if __has_include("/home/yuruhiya/contest/library/dump.hpp")
#include "/home/yuruhiya/contest/library/dump.hpp"
#else
#define dump(...) ((void)0)
#endif

// ---------------------------------------------------------------- //

int main() {
	int n = in;
	VL a = in[n];
	ll ans = 0;
	FOR(i, 1, n) {
		if (a[i - 1] > a[i]) {
			ans += a[i - 1] - a[i];
			a[i] = a[i - 1];
		}
	}
	out(ans);
}