#include <iostream>
#include <vector>
#include <iomanip>


using Matrix = std::vector<std::vector<int>>;


double result(double time, int quantityOfSegments, Matrix& arrayOfTimesAndSpeeds) {
    double answer{}, now{};

    for (int i{}; i < quantityOfSegments; ++i) {
        if (now + arrayOfTimesAndSpeeds[0][i] < time) {
            now += arrayOfTimesAndSpeeds[0][i];
            answer += arrayOfTimesAndSpeeds[1][i] * arrayOfTimesAndSpeeds[0][i];
        } else {
            answer += (time - now) * arrayOfTimesAndSpeeds[1][i];
            break;
        }
    }
    return answer;
}


bool isMiddleCorrect(double middle, int quantityOfSegments, Matrix& arrayOfTimesAndSpeeds, int length) {
    double temporaryTime{}, times{};

    for (int i{}; i < quantityOfSegments; ++i) {
        times += arrayOfTimesAndSpeeds[0][i];
    }

    for (int i{}; i < quantityOfSegments; ++i) {
        temporaryTime += arrayOfTimesAndSpeeds[0][i];

        if (temporaryTime >= middle) {
            double lengthFirst{ result(temporaryTime, quantityOfSegments, arrayOfTimesAndSpeeds) -
                                result(temporaryTime - middle, quantityOfSegments, arrayOfTimesAndSpeeds) };
            double lengthSecond{ result(temporaryTime + middle, quantityOfSegments, arrayOfTimesAndSpeeds) -
                                 result(temporaryTime, quantityOfSegments, arrayOfTimesAndSpeeds) };

            if (temporaryTime + middle > times) {
                lengthSecond = length;
            }

            if (lengthFirst < length || lengthSecond < length) {
                return false;
            }
        }
    }

    double possibleLength = result(middle, quantityOfSegments, arrayOfTimesAndSpeeds);

    if (possibleLength < length) {
        return false;
    }

    return true;
}


double search(int quantityOfSegments, Matrix& arrayOfTimesAndSpeeds, int length) {
    double right{}, left{};

    for (int i{}; i < quantityOfSegments; ++i) {
        right += arrayOfTimesAndSpeeds[0][i];
    }

    while (right - left > 0.0001) {
        double middle{ left + (right - left) / 2 };
         (isMiddleCorrect(middle, quantityOfSegments, arrayOfTimesAndSpeeds, length)) ? right = middle : left = middle;
    }

    return right;
}


int main() {
    int distance, quantityOfSegments, time, speed;
    std::cin >> distance >> quantityOfSegments;

    Matrix arrayOfTimesAndSpeeds(2);

    for (int i{}; i < 2; ++i) {
        arrayOfTimesAndSpeeds[i] = std::vector<int>(quantityOfSegments);
    }

    for (int i{}; i < quantityOfSegments; ++i) {
        std::cin >> time >> speed;

        arrayOfTimesAndSpeeds[0][i] = time;
        arrayOfTimesAndSpeeds[1][i] = speed;
    }


    std::cout << std::fixed << std::setprecision(3) << search(quantityOfSegments, arrayOfTimesAndSpeeds, distance);

    return 0;
}