#include <bits/stdc++.h>
using namespace std;
int n, centru;
int l[10];
int r[10];
int vars(int i, int tip) {
  if (!tip) {
    if (l[i] <= centru && centru <= r[i])
      return centru - l[i];
    else if (r[i] < centru)
      return r[i] - l[i] + 1;
    return 0;
  } else if (tip == 1)
    return ((bool)(l[i] <= centru && centru <= r[i]));
  else {
    if (l[i] <= centru && centru <= r[i])
      return r[i] - centru;
    else if (centru < l[i])
      return r[i] - l[i] + 1;
    return 0;
  }
}
int tip[10];
int _s[3];
double sum;
void backtr(int poz) {
  if (poz == (n + 1)) {
    if (!_s[1]) return;
    if (_s[2] >= 2) return;
    if (_s[2] == 0) {
      if (_s[1] == 1) return;
    }
    double prod = 1;
    for (int i = 1; i <= n; i++) prod *= vars(i, tip[i]);
    sum += centru * prod;
    return;
  }
  for (int i = 0; i < 3; i++) {
    _s[i]++;
    tip[poz] = i;
    backtr(poz + 1);
    _s[i]--;
  }
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> l[i] >> r[i];
  for (int i = 1; i <= 10000; i++) {
    centru = i;
    backtr(1);
  }
  for (int i = 1; i <= n; i++) {
    sum /= (r[i] - l[i] + 1);
  }
  cout << fixed << setprecision(12) << sum << '\n';
  return 0;
}
