#pragma GCC diagnostic ignored "-Wunused-variable" 
#include <bits/stdc++.h>
using namespace std;

#define BOOST
#ifdef BOOST
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
using namespace boost;
using ml = multiprecision::cpp_int;
using md = multiprecision::cpp_dec_float_100;
#endif

/***** type *****/
using ll = long long;
using ld = long double;
using pll = pair<long long, long long>;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vector<T>>;
template <class T> using vvvt = vector<vector<vector<T>>>;
/***** define *****/
#define all(c) (c).begin(), (c).end()            // begin to end
#define coutld cout << fixed << setprecision(10) // cout double
#define output(x) do { cout << x << endl; exit(0); } while(0)
#define rep(i, b, e) for (ll i = b; i < e; i++)  // repeat
#define repr(i, b, e) for (ll i = b; e < i; i--) // repeat reverse
#define fori(i, ...) if (ll i = -1) for(__VA_ARGS__) if (i++, 1)
#define each(i, e, c) fori (i, auto&& e: c)      // for each
/***** const value *****/
#define llong_max 9223372036854775807            // 9 * 10^18
#define ldbl_max 1.79769e+308                    // 1.7 * 10^308
#define pi 3.1415926535897932                    // 3.14 ...
/***** lambda *****/
auto Ceil = [] // if (a % b != 0) return a / b + 1;
(auto x) { return (ll)ceil(x); };
auto Count = [] // long long count value
(auto b, auto e, auto x) { return (ll)count(b, e, x); };
auto CtoL = [] // char to number
(auto c) { return (ll)c - (ll)'0'; };
auto LtoC = [] // number to char
(auto n) { return (char)('0' + n); };
auto Pow = [] // long long pow
(auto a, auto b) { return (ll)pow(a, b); };
auto Pow2 = [] // long long pow2
(auto n) { return (1LL << n); };
auto Pow10 = [] // long long pow10
(auto n) { return (ll)pow(10, n); };
auto Size = []  // long long collection size
(auto& c) { return (ll)(c).size(); };
auto Sum = [] // long long accumulate
(auto b, auto e) { return accumulate(b, e, 0LL); };

/***** operator *****/
template <class T, class S> pair<T, S> operator + (pair<T, S> l, pair<T, S> r)
{	// pair<T, S> + pair<T, S>
	return { l.first + r.first, l.second + r.second };
}
template <class T, class S> pair<T, S> operator - (pair<T, S> l, pair<T, S> r)
{	// pair<T, S> - pair<T, S>
	return { l.first - r.first, l.second - r.second };
}

/***** template *****/
template <class T> void MakeVVT
(ll ys, ll xs, vvt<T>& v, T fill = T())
{	// vector<vector<T>> resize + fill
	v.resize(ys); rep(y, 0, ys) v[y].resize(xs, fill);
}
template <class T> void MakeVVVT
(ll zs, ll ys, ll xs, vvvt<T>& v, T fill = T())
{	// vector<vector<vector<T>>> resize + fill
	v.resize(zs); rep(z, 0, zs) MakeVVT(ys, xs, v[z], fill);
}
template <class T> void InputVVT
(ll ys, ll xs, vvt<T>& v, T fix = T())
{	// input vector<vector<T>> (T != struct) + fix
	MakeVVT(ys, xs, v, fix);
	rep(y, 0, ys) rep(x, 0, xs) { cin >> v[y][x]; v[y][x] += fix; }
}
template <class T> void InputVVVT
(ll zs, ll ys, ll xs, vvvt<T>& v, T fix = T())
{	// input vector<vector<vector<T>>> (T != struct) + fix
	v.resize(zs); rep(z, 0, zs) InputVVT(ys, xs, v[z], fix);
}

/**************************************/
/********** BEGIN OF NYA LIB **********/
/**************************************/

namespace NyaGadget {}

namespace NyaGadget
{
	/*** ランレングス圧縮ライブラリ ***/

	template <class T> struct RunLengthEncodingResult
	{	// ランレングス圧縮の結果を格納する構造体
		long long begin = 0; // 区間開始位置
		long long end = 0;   // 区間終了位置
		std::vector<T> sub;  // 区間データ
	};

	template <> struct RunLengthEncodingResult<std::string>
	{	// ランレングス圧縮の結果を格納する構造体
		long long begin = 0; // 区間開始位置
		long long end = 0;   // 区間終了位置
		std::string sub;     // 区間データ
	};

	template <class T> struct DS_NyaaRLE
	{
		std::vector<RunLengthEncodingResult<T>> res;

		/**
		@brief 文字列のランレングス圧縮
		@param s 圧縮する文字列
		@param res 圧縮結果
		**/
		decltype(res)& Run(std::string& s)
		{
			res.push_back({ 0, 0, "" });
			for (long long i = 0; i < (long long)s.size(); i++)
			{
				if (i == (long long)s.size() - 1)
				{
					res.back().end = i + 1;
					res.back().sub = s.substr(res.back().begin, i + 1 - res.back().begin);
					return res;
				}
				if (s[i] != s[i + 1])
				{
					res.back().end = i + 1;
					res.back().sub = s.substr(res.back().begin, i + 1 - res.back().begin);
					res.push_back({ i + 1, 0, "" });
				}
			}
			return res;
		}

		/**
		@brief 配列のランレングス圧縮
		@param v 圧縮する配列
		@param res 圧縮結果
		**/
		decltype(res)& Run(std::vector<T>& v)
		{
			res.push_back({ 0, 0, std::vector<T>() });
			for (long long i = 0; i < (long long)v.size(); i++)
			{
				if (i == (long long)v.size() - 1)
				{
					res.back().end = i + 1;
					for (long long j = res.back().begin; j < i + 1; j++) res.back().sub.push_back(v[j]);
					return res;
				}
				if (!IsSame(v[i], v[i + 1]))
				{
					res.back().end = i + 1;
					for (long long j = res.back().begin; j < i + 1; j++) res.back().sub.push_back(v[j]);
					res.push_back({ i + 1, 0, std::vector<T>() });
				}
			}
			return res;
		}

		bool IsSame(T& l, T& r)
		{	// lとrが同値かどうか調べる
			return l == r;
		}
	};
}

/**************************************/
/*********** END OF NYA LIB ***********/
/**************************************/

using namespace NyaGadget;
//using mll = NT_ModLL< 1000000007 >;
//using mll = ModLL< 998244353 >;

int main(void)
{
	ll N; cin >> N;
	vt<ll> A(N); each(i, e, A) cin >> e;

	if (A[0] != 0) output(-1);
	rep(i, 1, N) if (A[i - 1] + 1 < A[i]) output(-1);

	vt<ll> test(N);
	each(i, e, A) test[i] = i - e;

	ll ans = 0;
	DS_NyaaRLE<ll> rle;
	auto res = rle.Run(test);
	each(i, e, res) ans += e.end - 1 - e.sub[0];
	cout << ans << endl;
	return 0;
}


Measured memory_bytes: 34332