#include <fstream>
using namespace std;

int main() {
    ifstream cin("lottery.in");
    ofstream cout("lottery.out");
    int n, m;
    double s = 0;
    cin >> n >> m;
    int p, val;
    cin >> p >> val;
    for (int i = 1; i < m; ++i) {
        int curP;
        cin >> curP;
        s += val * 1.0 / (p * curP) * (curP - 1);
        p *= curP;
        cin >> val;
    }
    s += val * 1.0 / p;
    cout << n - s;
    return 0;
}