static void Main()
{
var ss = new StreamScanner(new StreamReader(Console.OpenStandardInput()));
var sw = new StreamWriter(Console.OpenStandardOutput())
{
AutoFlush = false
};
new Program().Solve(ss, sw);
sw.Flush();
}
void Solve(StreamScanner ss, StreamWriter sw)
{
//---------------------------------
var N = ss.Next(int.Parse);
var A = ss.Next(long.Parse, N);
if (A[0] != 0)
{
sw.WriteLine(-1);
return;
}
var ans = 0L;
for (var i = 1; i < N; i++)
{
if (A[i] > A[i - 1] + 1)
{
sw.WriteLine(-1);
return;
}
if (A[i] == A[i - 1] + 1)
{
continue;
}
if (A[i] <= A[i - 1])
{
ans += A[i - 1];
}
}
ans += A[N - 1];
sw.WriteLine(ans);
//---------------------------------
}