Problem:
Score : 300 points

Problem Statement

You are given N real values A_1, A_2, \ldots, A_N.
Compute the number of pairs of indices (i, j)
such that i < j and the product A_i \cdot A_j is integer.

Constraints

2 \leq N \leq 200\,000

0 < A_i < 10^4

A_i is given with at most 9 digits after the decimal.

Input

Input is given from Standard Input in the following format.

N
A_1
A_2
\vdots
A_N

Output

Print the number of pairs with integer product A_i \cdot A_j (and i < j).

Sample Input 1

5
7.5
2.4
17.000000001
17
16.000000000

Sample Output 1

3

There are 3 pairs with integer product:

7.5 \cdot 2.4 = 18

7.5 \cdot 16 = 120

17 \cdot 16 = 272

Sample Input 2

11
0.9
1
1
1.25
2.30000
5
70
0.000000001
9999.999999999
0.999999999
1.000000001

Sample Output 2

8

Solution in C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;
using static System.Math;
 
public class Solve{
    static public int mod = 1000000007;
    static public string al = "abcdefghijklmnopqrstuvwxyz";
    public static void Main(){
        // •ûj
        //
        var n = rint();
        var counter = new int[20,20,20]; // 2 5 10^-n
        var count2 = new int[n];
        var count5 = new int[n];
        var count10 = new int[n];
        
        for(int i=0;i<n;i++){
            var s = rstr().Split('.');
            if(s.Length == 2){
                count10[i] = s[1].Length;
            }
            long num = 0;
            if(s.Length == 1){
                num = long.Parse(s[0]);
            }else{
                num = long.Parse(s[0]+s[1]);
            }
            while(num % 2 == 0){
                num/=2;
                count2[i]++;
            }
            while(num % 5 == 0){
                num/=5;
                count5[i]++;
            }
            
            while(count2[i] > 0 && count5[i] > 0 && count10[i] > 0){
                count2[i]--;
                count5[i]--;
                count10[i]--;
            }
            count2[i] = Min(19,count2[i]);
            count5[i] = Min(19,count5[i]);
            //WriteLine($"{count2[i]} {count5[i]} {count10[i]}");
            counter[count2[i],count5[i],count10[i]]++;
        }
        //WriteLine(string.Join(" ",count2));
        //WriteLine(string.Join(" ",count5));
        //WriteLine(string.Join(" ",count10));
        
        for(int i=0;i<20;i++){
            for(int j=0;j<20;j++){
                for(int k=0;k<9;k++){
                    counter[i,j,k+1] += counter[i,j,k];
                }
            }
        }
        /*
        for(int k=0;k<10;k++){
            WriteLine("---------------------------------------");
            for(int i=0;i<20;i++){
                for(int j=0;j<20;j++){
                    Write($"{counter[i,j,k]} ");
                }
                WriteLine();
            }
        }
        */
        
        long ans = 0;
        for(int i=0;i<n;i++){
            
            for(int j=0;j<20;j++){
                for(int k=0;k<20;k++){
                    var max10 = Min(19,Min(count2[i] + j,count5[i] + k) - count10[i]);
                    if(max10 < 0) continue;
                    ans += counter[j,k,max10];
                    //if(counter[j,k,max10] > 0) WriteLine($"{i} {j}/{k} {count2[i] + j}/{count5[i] + k}/{max10} {counter[j,k,max10]}");
                }
            }
            //WriteLine(ans);
            if(count10[i] == 0) ans--;
            
        }
        WriteLine(ans/2);
        
        
        
        
        
	
    }
    public static void swap(ref int a,ref int b){int temp = a;a= b;b = temp;}
    static void charswap(ref char a,ref char b){char temp = a;a= b;b = temp;}
    static int ncr(int n,int r){if(n<r)return 0;r = Min(r,n-r);long nn = 1;for(int i=n-r+1;i<=n;i++){nn = nn*i%mod;}long rr = 1;for(int i=1;i<=r;i++){rr = rr*i%mod;}rr = square((int)rr,mod-2);nn = nn * rr %mod;return (int)nn;}
    // a^b mod
    static int square(int a,int b){string binary = Convert.ToString(b,2);int bileng = binary.Length;long a_power = a;long value = 1;for(int i=bileng-1;i>=0;i--){if(binary[i] == '1'){value = value*a_power%mod;}a_power = a_power*a_power%mod;}return (int)value;}
    static int square2(int a,int b){long output = 1;var list = new List<long>();int sh = 1;long n = a;list.Add(a);while(sh < b){sh *= 2;n = n*n%mod;list.Add(n);}for(int i=list.Count-1;i>=0;i--){if(b > sh){b -= sh;sh /= 2;output = output*list[i]%mod;}}return (int)output;}
    //Šeí“Çæ
    static string rstr(){ return ReadLine(); }
    static int rint(){ return int.Parse(ReadLine()); }
    static long rlong(){ return long.Parse(ReadLine()); }
    static string[] stra(){ return ReadLine().Split(' '); }
    static char[] chara(){ string[] a=stra();string b="";for(int i=0;i<a.Length;i++){b+=a[i];}return b.ToCharArray();}
    static int[] inta(){ string[] read_str_array = ReadLine().Split(' '); int[] return_int_array = new int[read_str_array.Length]; for(int countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_int_array[countup_i] = int.Parse(read_str_array[countup_i]); } return return_int_array; }
    static int[,] inta2(int num_array,int in_array){ int[,] int_array2 = new int[num_array,in_array]; for(int i=0;i<num_array;i++){ int[] temp_array = inta(); for(int j=0;j<in_array;j++){ int_array2[i,j] = temp_array[j]; } } return int_array2; }
    static long[] longa(){ string[] read_str_array = ReadLine().Split(' '); long[] return_long_array = new long[read_str_array.Length]; for(long countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_long_array[countup_i] = long.Parse(read_str_array[countup_i]); } return return_long_array; }
    static double[] doublea(){ string[] read_str_array = ReadLine().Split(' '); double[] return_double_array = new double[read_str_array.Length]; for(long countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_double_array[countup_i] = double.Parse(read_str_array[countup_i]); } return return_double_array; }
    // -----------------------------
    static long GCD(long a,long b){ if(a < b){ long temp = a; a = b; b = temp; } if(a % b == 0){ return b; } else{ long temp = b; b = a%b; a = temp; return GCD(a,b); } }
    static long LCM(long a,long b){ return a * b / GCD(a,b); }
    static void WriteArray(int[,] a,int b,int c){for(int i=0;i<b;i++){for(int j=0;j<c;j++){if(j!=0) Write(" ");Write(a[i,j]);}WriteLine();}}
}