#define FNAME "trains"
#define _CRT_SECURE_NO_WARNINGS
#define fi first
#define se second
#include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>

using namespace std;

typedef pair<double, double> pdd;

const int N = 1e4;
const double BIN = 1e7 + 7;

int t[N], v[N];
vector<pdd> a(N);

int find_lower(vector<pdd> &a, pair<double, double> val) {
    int l = 0, r = a.size();

    while (r - l > 1) {
        int m = (l + r) / 2;
        a[m].first <= val.first ? l = m : r = m;
    }

    return l;
}

int find_upper(vector<pdd> &a, pair<double, double> val) {
    int l = -1, r = a.size() - 1;

    while (r - l > 1) {
        int m = (l + r) / 2;
        a[m].first < val.first ? l = m : r = m;
    }

    return r;
}

int main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#else
    freopen(FNAME".in", "r", stdin);
    freopen(FNAME".out", "w", stdout);
#endif
    ios::sync_with_stdio(false);

    a.clear();
    a.push_back(make_pair(0, 0));

    int l, n;
    cin >> l >> n;
    for (int i = 0; i < n; ++i) {
        cin >> t[i] >> v[i];
        a.push_back(make_pair(a.back().fi + t[i], a.back().se + t[i] * v[i]));
    }

    double bin_l = 0, bin_r = BIN;
    for (int it = 0; it < 1000; ++it) {
        double m = (bin_l + bin_r) / 2;

        bool crushed = false;
        for (int k = 0; k < n; ++k) {
            pdd j = a[k];
            j.fi += m;

            int i = find_lower(a, j);
            if (i == n) {
                break;
            }
            if ((a[i].se + ((j.fi - a[i].fi) * (double)v[i]) - j.se) < (double)l) {
                crushed = true;
                break;
            }
        }
        for (int k = n; k > 0; --k) {
            pdd i = a[k];
            i.fi -= m;

            int j = find_upper(a, i);
            if (j == 0 && i.fi < a[j].fi) {
                break;
            }
            if ((i.se + ((a[j].fi - i.fi) * (double)v[max(0, j - 1)]) - a[j].se) < (double)l) {
                crushed = true;
                break;
            }
        }

        crushed ? bin_l = m : bin_r = m;
    }

    cout << setprecision(3) << fixed << bin_r << "\n";
    return 0;
}