#include<bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); i++)
#define R(i, j, k) for(int i = (j); i >= (k); i--)
#define ll long long
#define ull unsigned long long 
#define sz(a) ((int) a.size())
#define vi vector<int>
#define me(a, x) memset(a, x, sizeof(a))
using namespace std;
const int N = 1 << 21, L = 5e5, mod = 998244353;

double f[N];

ll n;

int k;
ll a[N], b[N];
ll rd() {
	cin >> k;
	L(i, 1, k) {
		cin >> a[i] >> b[i];
	}
	n = 0;
	while(true) {
		int win = 1;
		L(i, 1, k) {
			if(n % b[i] != a[i]) {
				win = 0;
				break;
			}
		}
		if(win) break;
		++n;
		if(n > 200) {
			break;
		}
	}
	return n;
}

double slv(vector < double > a, int cur) { 
	if(sz(a) == 2) {
		return cur == 0;
	}
	double win = 1;
	L(p, 1, sz(a) - 1) if(p != cur) {
		double sum1 = 0, sum2 = 0;
		L(i, 0, sz(a) - 1) if(i != cur) {
			sum1 += a[i];
			if(i != p) sum2 += a[i];
		}
		sum1 /= sum2;
		vector < double > b;
		L(i, 0, sz(a) - 1) {
			if(i == cur) b.emplace_back(a[i]);
			else if(i != p) b.emplace_back(a[i] * sum1);
		}
		
		double mx = 0;
		L(i, 0, sz(b) - 1) {
			mx = max(mx, b[i]);
		}
		
		int total = 0;
		double sum = 0;
		L(i, 0, sz(b) - 1) {
			if(fabs(b[i] - mx) <= 1e-9) {
				total += 1;
				sum += slv(b, i);
			}
		}
		
		sum /= total;
		
		win = min(win, sum);
	}
	return win;
}

int main () { 
	ios :: sync_with_stdio(false);
	cin.tie (0); cout.tie (0);
	n = rd();
	if(n < 2) {
		cout << "error\n";
		return 0;
	}
	vector < double > f(n);
	L(i, 0, n - 1) {
		f[i] = 1;
	}
	double ans = (slv(f, 0) + slv(f, 1) * (n - 1)) / n;
	cout.precision(6); cout << fixed;
	cout << ans << '\n';
	return 0;
}

/*
(n-1)/(n-2) 1 [(n-1)/(n-2)] (n-1)/(n-2) (n-1)/(n-2) x 
*/