#include <iostream>
#include <vector>
#include <fstream>
#include <cmath>
#include <limits>
using namespace std;
vector <int> t, v, x;
int mt = 0, l;
 
double dx(double rt) {
	double dx = 0, dt = 0, i = 0;
	bool f = true;
    while (f) {
        if ((i > 0 && dt + t[i] - t[i-1] >= rt) || (dt + t[i] >= rt && !i)){
            dx += v[i] * (rt - dt);
            f = false;
        } else {
        	dt += (i)?t[i] - t[i-1]:t[0];
        	dx += x[i++];
        }
    }
    return dx;
}
 
int check(double m) {
	double mn = numeric_limits<double>::max();
    if (dx(m) < l) return 0;
    for (int i = 0; i < t.size(); i++){
        if (t[i] >= m){
            double s1 = double(x[i] - dx(t[i]-m));
            double s2 = (t[i] + m <= mt)?double(dx(t[i]+m) - x[i]):l;
            mn = min(mn, min(s1, s2));
        }
    }
    if (mn < l) return false;
    else return true;
}
 
double srch(double l, double r) {
	while (r - l > 0.00001) {
		double mid = 1. * (l + r) / 2;
		if (check(mid)) r = mid;
		else l = mid;
	}
	return r;
}
 
int main() {
	ifstream cin("trains.in");
	ofstream cout("trains.out");
	int n, a, b;
	double at = 0;
	cin >> l >> n;
	for (int i = 0; i < n; ++i) {
		cin >> a >> b;
		t.push_back(a);
		v.push_back(b);
		x.push_back(t[i] * v[i]);
		if (i) {
			t[i] += t[i-1];
			x[i] += x[i-1];
		}
	}
	mt = t[n-1];
	cout << srch(0, double(mt)) << endl;
}

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1000
4
10 0
30 80
15 0
20 100