#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <numeric>
#include <cmath>

using namespace std;

bool checkTiming(double timing, double maxTiming, vector<int> timings,
        vector<int> speeds, int l) {
    double train1 = 0.0;
    double train2 = 0.0;
    double time1 = 0.0;
    double time2 = 0.0;
    int train1Step = 0;
    double firstTiming = timing;
    while (firstTiming > 0) {
        firstTiming -= (double) timings[train1Step];
        train1Step++;
    }
    firstTiming += (double) timings[--train1Step];
    for (int i = 0; i < train1Step; i++) {
        train1 += (double) timings[i] * (double) speeds[i];
        time1 += (double) timings[i];
    }
    train1 += (double) firstTiming * (double) speeds[train1Step];
    int i = 0;
    int it1 = i + train1Step;
    int it2 = i;
    time1 += firstTiming;
    double currentTime1 = firstTiming;
    double currentTime2 = 0.0;
    if (train1 - train2 < (double) l) {
        return false;
    }
    while (time1 < maxTiming) {
        double extraTime1 = (double) timings[it1] - currentTime1;
        train1 += extraTime1 * (double) speeds[it1++];
        time1 += extraTime1;
        while (extraTime1 > 0) {
            if ((double) timings[it2] - currentTime2 < extraTime1) {
                time2 += (double) timings[it2] - currentTime2;
                train2 += ((double) timings[it2] - currentTime2)
                        * (double) speeds[it2];
                extraTime1 -= (double) timings[it2] - currentTime2;
                currentTime2 = 0.0;
                it2++;
            } else {
                time2 += extraTime1;
                currentTime2 += extraTime1;
                train2 += extraTime1 * (double) speeds[it2];
                extraTime1 = 0.0;
            }
        }
        if (train1 - train2 < (double) l)
            return false;
        currentTime1 = 0.0;
        //cout << time1 << " " << time2 << " " << train1 << " " << train2 << endl;
    }
    return true;
}

double binsearch(double& left, double& right, vector<int> timings,
        double maxTiming, vector<int> speeds, int l) {
    double c = right;
    while (right - left > 0.001) {
        double mid = (right + left) / 2;
        //cout << mid << endl;
        bool v = checkTiming(mid, maxTiming, timings, speeds, l);
        if (v) {
            if (mid < c)
                c = mid;
            right = mid;
        } else {
            left = mid;
        }
    }
    return (right + left) / 2;
}
int main() {
    ifstream reader("trains.in");
    ofstream writer("trains.out");
    int l, n;
    reader >> l >> n;
    vector<int> timings;
    vector<int> speeds;
    for (int i = 0; i != n; i++) {
        int timing;
        int speed;
        reader >> timing;
        reader >> speed;
        timings.push_back(timing);
        speeds.push_back(speed);
    }
    double time = (double) accumulate(timings.begin(), timings.end(), 0.0);
    double left = 0.0;
    double value;
    value = binsearch(left, time, timings, time, speeds, l);
    writer << fixed << setprecision(3) << value;
    //cout<<endl;
    //cout << checkTiming (27.5, time, timings, speeds, l);
    writer.close();
    return 0;
}