#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;

    vector<pair<int, int>> interval(N);
    for (int i = 0; i < N; i++) cin >> interval[i].first >> interval[i].second;

    double ans = 1.0;
    for (int i = 1; i < N; i++) {
        auto [li, ri] = interval[i];
        auto [l0, r0] = interval[0];
        int l = min(max(li, l0), r0);
        int r = min(ri, r0);

        double t = l-l0;
        if (l <= r) t += 0.5*(2*ri-l-r)*(r-l+1)/(ri-li+1);
        t /= r0-l0+1;
        
        ans += t;
    }

    cout << fixed << setprecision(16) << ans << endl;
}