func_code_string: #include <bits/stdc++.h>   using namespace std;   const double PI = acos(-1);   const double eps = 1e-9;   const int inf = 2000000000;   const long long infLL = 9000000000000000000;   inline bool checkBit(long long n, int i) { return n & (1LL << i); }   inline long long setBit(long long n, int i) {     return n | (1LL << i);     ;   }   inline long long resetBit(long long n, int i) { return n & (~(1LL << i)); }   int dx[] = {0, 0, +1, -1};   int dy[] = {+1, -1, 0, 0};   inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }   inline bool isLeapYear(long long year) {     return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);   }   inline void normal(long long &a) {     a %= 1000000007;     (a < 0) && (a += 1000000007);   }   inline long long modMul(long long a, long long b) {     a %= 1000000007, b %= 1000000007;     normal(a), normal(b);     return (a * b) % 1000000007;   }   inline long long modAdd(long long a, long long b) {     a %= 1000000007, b %= 1000000007;     normal(a), normal(b);     return (a + b) % 1000000007;   }   inline long long modSub(long long a, long long b) {     a %= 1000000007, b %= 1000000007;     normal(a), normal(b);     a -= b;     normal(a);     return a;   }   inline long long modPow(long long b, long long p) {     long long r = 1;     while (p) {       if (p & 1) r = modMul(r, b);       b = modMul(b, b);       p >>= 1;     }     return r;   }   inline long long modInverse(long long a) { return modPow(a, 1000000007 - 2); }   inline long long modDiv(long long a, long long b) {     return modMul(a, modInverse(b));   }   const int MX = 10100;   int vis[5][5][5], l[5], r[5], curnum, n;   double dp[5][5][5];   double func(int pos, int eNum, int bNum) {     if (bNum > 1) return 0;     if (pos == n) {       if (eNum > 1 && !bNum) return 1;       if (eNum && bNum) return 1;       return 0;     }     if (vis[pos][eNum][bNum]) return dp[pos][eNum][bNum];     vis[pos][eNum][bNum] = 0;     double &ret = dp[pos][eNum][bNum];     ret = 0;     if (curnum >= l[pos] && curnum <= r[pos]) {       double temp = (1.0 / (r[pos] - l[pos] + 1));       temp *= func(pos + 1, eNum + 1, bNum);       ret += temp;     }     if (r[pos] > curnum) {       double temp = 1;       if (l[pos] <= curnum) {         temp *= (double)(r[pos] - curnum) / (double)(r[pos] - l[pos] + 1);       }       temp *= func(pos + 1, eNum, bNum + 1);       ret += temp;     }     if (l[pos] < curnum) {       double temp = 1;       if (r[pos] >= curnum) {         temp *= (double)(curnum - l[pos]) / (double)(r[pos] - l[pos] + 1);       }       temp *= func(pos + 1, eNum, bNum);       ret += temp;     }     return ret;   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     ;     cout.unsetf(ios::floatfield);     cout.precision(10);     cout.setf(ios::fixed, ios::floatfield);     ;     cin >> n;     for (int i = 0; i < n; ++i) cin >> l[i] >> r[i];     double ans = 0;     for (int i = 1; i < MX; ++i) {       curnum = i;       ans += i * func(0, 0, 0);       ;       memset(vis, 0, sizeof(vis));     }     cout << ans;     return 0;   }   