Language: C

#include  // Integer Product, by Errichto
using namespace std;
int main() {
    int n;
    scanf("%d", &n);
    map, int> cnt;
    for(int rep = 0; rep < n; ++rep) {
        double x;
        scanf("%lf", &x);
        long long i = llround(x  1e9);
        int two = 0, five = 0;
        while(i % 2 == 0) {
            two++;
            i /= 2;
        }
        while(i % 5 == 0) {
            five++;
            i /= 5;
        }
        ++cnt[{two,five}];
    }
    long long answer = 0;
    for(auto p1 : cnt) {
        for(auto p2 : cnt) {
            if(p1.first.first+p2.first.first >= 18 && p1.first.second+p2.first.second >= 18) {
                if(p1 < p2) {
                    answer += (long long) p1.second  p2.second;
                }
                else if(p1 == p2) {
                    answer += (long long) p1.second * (p1.second - 1) / 2;
                }
            }
        }
    }
    printf("%lld\n", answer);
}

Compiler output:
main.c:1:42: error: #include expects "FILENAME" or <FILENAME>
    1 | #include  // Integer Product, by Errichto
      |                                          ^
main.c:2:1: error: unknown type name 'using'
    2 | using namespace std;
      | ^~~~~
main.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    2 | using namespace std;
      |                 ^~~
main.c: In function 'main':
main.c:5:5: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    5 |     scanf("%d", &n);
      |     ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | #include  // Integer Product, by Errichto
main.c:5:5: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    5 |     scanf("%d", &n);
      |     ^~~~~
main.c:5:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:6:5: error: 'map' undeclared (first use in this function)
    6 |     map, int> cnt;
      |     ^~~
main.c:6:5: note: each undeclared identifier is reported only once for each function it appears in
main.c:6:10: error: expected expression before 'int'
    6 |     map, int> cnt;
      |          ^~~
main.c:10:23: error: implicit declaration of function 'llround' [-Wimplicit-function-declaration]
   10 |         long long i = llround(x  1e9);
      |                       ^~~~~~~
main.c:1:1: note: include '<math.h>' or provide a declaration of 'llround'
  +++ |+#include <math.h>
    1 | #include  // Integer Product, by Errichto
main.c:10:23: warning: incompatible implicit declaration of built-in function 'llround' [-Wbuiltin-declaration-mismatch]
   10 |         long long i = llround(x  1e9);
      |                       ^~~~~~~
main.c:10:23: note: include '<math.h>' or provide a declaration of 'llround'
main.c:10:32: error: expected ')' before numeric constant
   10 |         long long i = llround(x  1e9);
      |                              ~ ^ ~~~
      |                                )
main.c:20:11: error: 'cnt' undeclared (first use in this function); did you mean 'int'?
   20 |         ++cnt[{two,five}];
      |           ^~~
      |           int
main.c:20:15: error: expected expression before '{' token
   20 |         ++cnt[{two,five}];
      |               ^
main.c:23:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before ':' token
   23 |     for(auto p1 : cnt) {
      |                 ^
main.c:24:21: error: expected '=', ',', ';', 'asm' or '__attribute__' before ':' token
   24 |         for(auto p2 : cnt) {
      |                     ^
main.c:25:16: error: 'p1' undeclared (first use in this function)
   25 |             if(p1.first.first+p2.first.first >= 18 && p1.first.second+p2.first.second >= 18) {
      |                ^~
main.c:25:31: error: 'p2' undeclared (first use in this function)
   25 |             if(p1.first.first+p2.first.first >= 18 && p1.first.second+p2.first.second >= 18) {
      |                               ^~
main.c:27:52: error: expected ';' before 'p2'
   27 |                     answer += (long long) p1.second  p2.second;
      |                                                    ^ ~~
      |                                                    ;
main.c:35:5: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   35 |     printf("%lld\n", answer);
      |     ^~~~~~
main.c:35:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:35:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:35:5: note: include '<stdio.h>' or provide a declaration of 'printf'