Language: C++

#include<iostream>
#include<vector>
using namespace std;

typedef ULL unsigned long long;

int main()
{
	ULL N, M, L;
	cin >> N >> M >> L;
	
	vector<vector<ULL>> matrix(N, vector<ULL>(M));
	for(auto it = matrix.begin(); it != matrix.end(); it++)
	{
		for(auto itx = it->begin(); itx != it->end(); itx++)
		{
			cin >> *itx;
		}
	}
	
	vector<vector<ULL>> matrix_B(M, vector<ULL>(L));
	for(auto it = matrix_B.begin(); it != matrix_B.end(); it++)
	{
		for(auto itx = it->begin(); itx != it->end(); itx++)
		{
			cin >> *itx;
		}
	}
	
	for(int i = 0; i < N; i++)
	{
		for(int j = 0; j < L; j++)
		{
			ULL ans = 0;
			
			for(int k = 0; k < M; k++)
			{
				ans += matrix[i][k] * matrix_B[k][j];
			}
			
			cout << ans;
			if(j + 1 != L)cout << " ";
		}
		cout << endl;
	}
	
	return 0;
}

Compiler output:
a.cc:5:9: error: 'ULL' does not name a type; did you mean 'NULL'?
    5 | typedef ULL unsigned long long;
      |         ^~~
      |         NULL
a.cc: In function 'int main()':
a.cc:9:9: error: 'ULL' was not declared in this scope; did you mean 'NULL'?
    9 |         ULL N, M, L;
      |         ^~~
      |         NULL
a.cc:10:16: error: 'N' was not declared in this scope
   10 |         cin >> N >> M >> L;
      |                ^
a.cc:10:21: error: 'M' was not declared in this scope
   10 |         cin >> N >> M >> L;
      |                     ^
a.cc:10:26: error: 'L' was not declared in this scope
   10 |         cin >> N >> M >> L;
      |                          ^
a.cc:12:23: error: template argument 2 is invalid
   12 |         vector<vector<ULL>> matrix(N, vector<ULL>(M));
      |                       ^~~
a.cc:12:26: error: template argument 1 is invalid
   12 |         vector<vector<ULL>> matrix(N, vector<ULL>(M));
      |                          ^~
a.cc:12:26: error: template argument 2 is invalid
a.cc:12:49: error: template argument 2 is invalid
   12 |         vector<vector<ULL>> matrix(N, vector<ULL>(M));
      |                                                 ^
a.cc:12:53: error: expression list treated as compound expression in initializer [-fpermissive]
   12 |         vector<vector<ULL>> matrix(N, vector<ULL>(M));
      |                                                     ^
a.cc:13:30: error: request for member 'begin' in 'matrix', which is of non-class type 'int'
   13 |         for(auto it = matrix.begin(); it != matrix.end(); it++)
      |                              ^~~~~
a.cc:13:52: error: request for member 'end' in 'matrix', which is of non-class type 'int'
   13 |         for(auto it = matrix.begin(); it != matrix.end(); it++)
      |                                                    ^~~
a.cc:21:23: error: template argument 2 is invalid
   21 |         vector<vector<ULL>> matrix_B(M, vector<ULL>(L));
      |                       ^~~
a.cc:21:26: error: template argument 1 is invalid
   21 |         vector<vector<ULL>> matrix_B(M, vector<ULL>(L));
      |                          ^~
a.cc:21:26: error: template argument 2 is invalid
a.cc:21:51: error: template argument 2 is invalid
   21 |         vector<vector<ULL>> matrix_B(M, vector<ULL>(L));
      |                                                   ^
a.cc:21:55: error: expression list treated as compound expression in initializer [-fpermissive]
   21 |         vector<vector<ULL>> matrix_B(M, vector<ULL>(L));
      |                                                       ^
a.cc:22:32: error: request for member 'begin' in 'matrix_B', which is of non-class type 'int'
   22 |         for(auto it = matrix_B.begin(); it != matrix_B.end(); it++)
      |                                ^~~~~
a.cc:22:56: error: request for member 'end' in 'matrix_B', which is of non-class type 'int'
   22 |         for(auto it = matrix_B.begin(); it != matrix_B.end(); it++)
      |                                                        ^~~
a.cc:34:28: error: expected ';' before 'ans'
   34 |                         ULL ans = 0;
      |                            ^~~~
      |                            ;
a.cc:38:33: error: 'ans' was not declared in this scope; did you mean 'abs'?
   38 |                                 ans += matrix[i][k] * matrix_B[k][j];
      |                                 ^~~
      |                                 abs
a.cc:38:46: error: invalid types 'int[int]' for array subscript
   38 |                                 ans += matrix[i][k] * matrix_B[k][j];
      |                                              ^
a.cc:38:63: error: invalid types 'int[int]' for array subscript
   38 |                                 ans += matrix[i][k] * matrix_B[k][j];
      |                                                               ^
a.cc:41:33: error: 'ans' was not declared in this scope; did you mean 'abs'?
   41 |                         cout << ans;
      |                                 ^~~
      |                                 abs