#include <bits/stdc++.h>   using namespace std;   int n, i, j, k, e, x, l[7], r[7];   double p, c, res, f[7][7][3];   int main() {     scanf( %d , &n);     for (i = 0; i < n; i++) scanf( %d%d , &l[i], &r[i]);     for (i = 1; i <= 10000; i++) {       memset(f, 0, sizeof(f));       f[0][0][0] = 1;       for (j = 0; j < n; j++) {         p = 1. / (r[j] - l[j] + 1.);         for (k = 0; k <= j; k++)           for (e = 0; e < 2; e++)             if (f[j][k][e]) {               x = min(r[j] + 1, i) - l[j];               if (x > 0) f[j + 1][k][e] += f[j][k][e] * x * p;               if (i >= l[j] && i <= r[j]) f[j + 1][k + 1][e] += f[j][k][e] * p;               x = r[j] - max(l[j] - 1, i);               if (x > 0) f[j + 1][k][e + 1] += f[j][k][e] * x * p;             }       }       for (c = 0, k = 1; k <= j; k++)         for (e = 0; e < 2; e++)           if ((k > 1 && e == 0) || e == 1) c += f[n][k][e];       res += c * i;     }     printf( %.15lf n , res);     return 0;   }   