#define _USE_MATH_DEFINES
#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <string>
#include <vector>
#include <utility>
#include <complex>
#include <set>
#include <map>
#include <unordered_map>
#include <queue>
#include <stack>
#include <deque>
#include <tuple>
#include <bitset>
#include <limits>
#include <algorithm>
#include <array>
#include <random>
#include <complex>
#include <regex>
using namespace std;
typedef long double ld;
typedef long long ll;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
typedef complex<ld> compd;
#define quickIO()	{cin.tie(0);	cout.sync_with_stdio(false);}
#define reach(i,a)	for(auto i:a)
#define rep(i,n)	for(int i=0;i<((int)n);i++)
#define REP(i,n)	for(int i=0;i<=((int)n);i++)
#define srep(i,a,n)	for(int i=a;i<((int)n);i++)
#define SREP(i,a,n)	for(int i=a;i<=((int)n);i++)
#define rrep(i,n)	for(int i=n-1;i>=0;i--)
#define RREP(i,n)	for(int i=n;i>=0;i--)
#define all(a)	(a).begin(),(a).end()
#define mp(a,b)	make_pair(a,b)
#define mt	make_tuple
#define pb	push_back
template<typename T> istream& operator >> (istream& is, vector<T>& vec) {
	for (T& x : vec)	is >> x;
	return is;
}
template<typename T> ostream& operator << (ostream& os, vector<T>& vec) {
	os << "[";
	rep(i, vec.size())	os << (i ? ", " : "") << vec[i];
	os << "]";
	return os;
}
template<typename T> istream& operator >> (istream& is, pair<T, T>& p) {
	is >> p.first >> p.second;
	return is;
}
template<typename T> ostream& operator << (ostream& os, pair<T, T>& p) {
	os << "(" << p.first << ", " << p.second << ")";
	return os;
}
int bitcnt(ll x) {
	x = ((x & 0xAAAAAAAAAAAAAAAA) >> 1) + (x & 0x5555555555555555);
	x = ((x & 0xCCCCCCCCCCCCCCCC) >> 2) + (x & 0x3333333333333333);
	x = ((x & 0xF0F0F0F0F0F0F0F0) >> 4) + (x & 0x0F0F0F0F0F0F0F0F);
	x = ((x & 0xFF00FF00FF00FF00) >> 8) + (x & 0x00FF00FF00FF00FF);
	x = ((x & 0xFFFF0000FFFF0000) >> 16) + (x & 0x0000FFFF0000FFFF);
	x = ((x & 0xFFFFFFFF00000000) >> 32) + (x & 0x00000000FFFFFFFF);
	return x;
}
int bitcnt(int x) {
	x = ((x & 0xAAAAAAAA) >> 1) + (x & 0x55555555);
	x = ((x & 0xCCCCCCCC) >> 2) + (x & 0x33333333);
	x = ((x & 0xF0F0F0F0) >> 4) + (x & 0x0F0F0F0F);
	x = ((x & 0xFF00FF00) >> 8) + (x & 0x00FF00FF);
	x = ((x & 0xFFFF0000) >> 16) + (x & 0x0000FFFF);
	return x;
}
ll sqrtll(ll x) {
	ll left = 0, right = x;
	rep(i, 100) {
		ll mid = (left + right) >> 1;
		if (mid*mid <= x)	left = mid;
		else	right = mid;
	}
	return left;
}
ll gcd(ll a, ll b) {
	return b == 0 ? a : gcd(b, a%b);
}
#define debug(x)	printf("Case #%d: ", x)
#define DEBUG 0
const ll inf = 1e9;
const ld infl = 1e100;
const ll mod = 1e9 + 7;
const ld eps = 1e-9;
const int dx[] = { 1,0,-1,0,0 };
const int dy[] = { 0,1,0,-1,0 };

ll segtree[4000010];
int N;

void init(int n) {
	N = 1;
	while (N < n)	N <<= 1;
}

void update(int k, int a) {
	k += N;
	while (k) {
		segtree[k] = a;
		k >>= 1;
	}
}

ll getMin(int l, int r, int a, int b, int k) {
	if (r <= a || b <= l) {
		return inf;
	}
	else if (l <= a && b <= r) {
		return segtree[k];
	}
	ll left = getMin(l, r, a, (a + b) / 2, 2 * k);
	ll right = getMin(l, r, (a + b) / 2, b, 2 * k + 1);
	return min(left, right);
}

int main() {
	int n;	cin >> n;
	vll a(n);
	rep(i, n)	cin >> a[i];
	// 作成できるかの判定
	// a[i]がある時点で、i-a[i]>0, a[i-a[i]+j]>=j(0<=j<=a[i])が条件
	// min(a[i-a[i]+j]-j|0<=j<=a[i]) = i-a[i]+min(a[j]-j|i-a[i]<=j<=i) >= 0が条件
	// a[i]の値を0-index換算したものの最小値をsegtreeに入れれば計算可能
	// 作成できると判明したら、後ろからa[i]+1==a[i+1]となるものをまとめて処理、
	// 他は個別で作るとしてゴリゴリするだけ
	init(n);
	rep(i, n) {
		update(i, a[i] - i);
	}
	rep(i, n) {
		if (i < a[i]) {
			cout << -1 << endl;
			return 0;
		}
		ll sml = getMin(i - a[i], i + 1, 0, N, 1);
		if (sml + i < a[i]) {
			cout << -1 << endl;
			return 0;
		}
	}
	// 構築
	ll ret = 0;
	rrep(i, n - 1) {
		if (a[i] + 1 == a[i + 1]) {
			ret++;
		}
		else {
			ret += a[i + 1];
		}
	}
	cout << ret << endl;
	return 0;
}

Measured memory_bytes: 5504