void main() {
	auto N = ri;
	auto A = iota(N).map!(a => ri).array;
	if(A[0] > 0) {
		writeln(-1);
		return;
	}
	foreach(i; 0..N-1) {
		if(A[i+1]-A[i] > 1) {
			writeln(-1);
			return;
		}
	}
	ulong ans;
	foreach(i; 0..N-1) {
		if(A[i]+1 == A[i+1]) ans++;
		else ans += A[i+1];
	}
	ans.writeln;
}

// ===================================

import std.stdio;
import std.string;
import std.conv;
import std.algorithm;
import std.range;
import std.traits;
import std.math;
import std.container;
import std.bigint;
import std.numeric;
import std.conv;
import std.typecons;
import std.uni;
import std.ascii;
import std.bitmanip;
import core.bitop;

T readAs(T)() if (isBasicType!T) {
	return readln.chomp.to!T;
}
T readAs(T)() if (isArray!T) {
	return readln.split.to!T;
}

T[][] readMatrix(T)(uint height, uint width) if (!isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		res[i] = readAs!(T[]);
	}
	return res;
}

T[][] readMatrix(T)(uint height, uint width) if (isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		auto s = rs;
		foreach(j; 0..width) res[i][j] = s[j].to!T;
	}
	return res;
}

int ri() {
	return readAs!int;
}

double rd() {
	return readAs!double;
}

string rs() {
	return readln.chomp;
}

T[] deepcopy(T)(T[] a) {
	static if(isArray!T) {
		T[] res;
		foreach(i; a) {
			res ~= deepcopy(i);
		}
		return res;
	} else {
		return a.dup;
	}
}

ulong[] generate_prime_list(T)(T N) if(isIntegral!T) {
	ulong[] prime_list = [2];
	bool not_prime = false;
	foreach(i; 3..N.to!ulong+1) {
		foreach(j; prime_list) {
			if(i % j == 0) {
				not_prime = true;
				break;
			}
		}
		if(!not_prime) prime_list ~= i;
		not_prime = false;
	}
	return prime_list;
}

bool isPrime(ulong n) {
	if(n <= 1) return false;
	else if(n == 2) return true;
	else if(n % 2 == 0) return false;

	foreach(i; iota(3, n.to!double.sqrt.ceil+1, 2)) {
		if(n % i == 0) return false;
	}
	return true;
}

class UnionFind(T) {
	T[] arr;
	this(ulong n) {
		arr.length = n+1;
		arr[] = -1;
	}
	T root(T x) {
		return arr[x] < 0 ? x : root(arr[x]);
	}
	bool same(T x, T y) {
		return root(x) == root(y);
	}
	bool unite(T x, T y) {
		x = root(x);
		y = root(y);
		if(x == y) return false;
		if(arr[x] > arr[y]) swap(x, y);
		arr[x] += arr[y];
		arr[y] = x;
		return true;
	}
	T size(T a) {
		return -arr[root(a)];
	}
}
