Parallel scheme to partition long calculation

ParallelFor.cs

async Task<double> ComputeStuffAsync(CancellationToken token)
		{
			var tsk = Task.Run(() =>
			{
				var sum = 0.0;
				int DOP = 4;

				Parallel.For(0, DOP - 1, new ParallelOptions { MaxDegreeOfParallelism = DOP, CancellationToken = token },
					// Initialize the local states
					() => (double)0.0,
					// Accumulate the thread-local computations in the loop body
					(thread, loop, localState) =>
					{
						long from = 1 + (thread * 100 * 1000 * 1000);
						long to = ((thread + 1) * 100 * 1000 * 1000);

						var computed = Calc(from, to);
						return localState + computed;
					},
					// Combine all local states
					localState => Interlocked.Exchange(ref sum, localState)
					);

				return sum;
			}, token);

			return await tsk;

			double Calc(long from, long to)
			{
				double sum = 0;
				for (long i = from; i < to; i++)
					sum += Math.Sqrt(i);
				return sum;
			}
		}