#include<iostream>
#include<vector>

using namespace std;

vector<int> L;
vector<int> R;
vector<int> A;
int n;
long double prob = 0;
int bound;
long double E = 0;

bool check()
{
    int lo = 0;
    int eq = 0;
    int hi = 0;
    for (int i = 0; i < n; ++i)
    {
        if (A[i] == 0)
        {
            if (R[i] < bound)
                return false;
            if (L[i] > bound)
                return false;
            eq += 1;
        }
        else if (A[i] > 0)
        {
            if (R[i] <= bound)
                return false;
            hi += 1;
        }
        else
        {
            if (L[i] >= bound)
                return false;
            lo += 1;
        }
    }
    if (hi == 1 && eq >= 1)
        return true;
    if (hi == 0 && eq >= 2)
        return true;
    return false;
}

long double calc_prob()
{
    long double ans = 1.0;
    long double ONE = 1.0;
    for (int i = 0; i < n; ++i)
    {
        if (A[i] == 0)
            ans *= ONE / (R[i] - L[i] + 1);
        else if (A[i] == 1)
            ans *= (long double)(R[i] - max(L[i], bound + 1) + 1) / (R[i] - L[i] + 1);
        else
            ans *= (long double)(min(R[i], bound - 1) - L[i] + 1) / (R[i] - L[i] + 1);
    }
    return ans;
}

int main()
{
    cin >> n;
    L.resize(n);
    R.resize(n);
    int i, m, j, d, d1;
    for (i = 0; i < n; ++ i)
    {
        cin >> L[i] >> R[i];
    }
    m = 1;
    for (i = 0; i < n; ++i)
        m *= 3;
    for (bound = 1; bound <= 10000; ++bound)
    {
        prob = 0;
        for (d = 0; d < m; ++d)
        {
            A.clear();
            d1 = d;
            for (i = 0; i < n; ++i)
            {
                A.push_back(d1 % 3 - 1);
                d1 /= 3;
            }
            if (check())
            {
                prob += calc_prob();
            }
        }
        E += bound * prob;
    }
    cout.precision(16);
    cout << (double) E << endl;
    return 0;
}