#include <algorithm>
#include <string>
#include <vector>
#include <iostream>
#include <iterator>
#include <numeric>
#include <utility>
#include <fstream>

#include <cmath>

int main(int argc, char** argv)
{
    if (argc < 2)
    {
	std::cerr << "Usage: " << argv[0] << " FILE" << std::endl;
	return 1;
    }

    std::ifstream ifs { argv[1] };
    if (!ifs)
    {
	std::cerr << "Unable to open file: " << argv[1] << std::endl;
	return 2;
    }

    double dx { };
    ifs >> dx;


    std::vector<double> values {
	std::istream_iterator<double>{ifs}, std::istream_iterator<double>{}
    };

    std::vector<double> lengths { };
    std::adjacent_difference(std::begin(values), std::end(values), std::back_inserter(lengths));
    std::transform(std::begin(lengths), std::end(lengths), std::begin(lengths),
		   [dx](double dy) { return std::sqrt(dx*dx + dy*dy); });
    
    double length { std::accumulate(std::next(std::begin(lengths)), std::end(lengths), 0.0) };

    std::cout << "The arc length is: " << length << std::endl;
}