Dijkstra's alghoritm with OpenMP - c++

Question

There is a fully working Dijkstra algorithm that takes values ​​from a file (or writes a new one) and writes data to a matrix. How can you improve the program using OpenMP so that its speed is significantly increased? New to openMP, had little experience with common matrices, mostly parallelization "for".
I attach all the code below:
#include <iostream>
#include <fstream>
#include <string>
#include <climits>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <iomanip>
#include <locale.h>
#include <chrono>
using namespace std;
class Timer
{
private:
using clock_t = std::chrono::high_resolution_clock;
using second_t = std::chrono::duration<double, std::ratio<1> >;
chrono::time_point<clock_t> m_beg;
public:
Timer() : m_beg(clock_t::now())
{
}
double elapsed() const
{
return std::chrono::duration_cast<second_t>(clock_t::now() - m_beg).count();
}
};
void fillingArray(int** arr, int c, string filename) {
ofstream fout(filename);
srand(static_cast<unsigned int>(time(0)));
for (int i = 0; i < c; i++){
for (int j = 0; j < c; j++) {
arr[i][j] = arr[j][i] = rand() % 10 + 1;
}
}
for (int i = 0; i < c; i++) {
fout << endl;
for (int j = 0; j < c; j++) {
fout << setw(5) << arr[i][j];
}
}
}
void read(int** arr, int c, string filename) {
char answer;
ifstream file;
file.open(filename);
if (!file) {
fillingArray(arr, c, filename);
}
else {
cout << "File found\nWant to overwrite it? (y / n)\n";
cin >> answer;
if (answer == 'y' || answer == 'Y') {
fillingArray(arr, c, filename);
}
else {
for (int i = 0; i < c; i++) {
for (int j = 0; j < c; j++) {
file >> arr[i][j];
}
}
}
}
file.close();
}
void Dijkstra(int** arr, int c, int st)
{
Timer t;
int count, index, i, u, m = st + 1;
int* distance = (int*)malloc(c * sizeof(int*));
bool* visited = new bool[c];
for (i = 0; i < c; i++)
{
distance[i] = INT_MAX; visited[i] = false;
}
distance[st] = 0;
for (count = 0; count < c - 1; count++)
{
int min = INT_MAX;
for (i = 0; i < c; i++)
if (!visited[i] && distance[i] <= min)
{
min = distance[i]; index = i;
}
u = index;
visited[u] = true;
for (i = 0; i < c; i++)
if (!visited[i] && arr[u][i] && distance[u] != INT_MAX &&
distance[u] + arr[u][i] < distance[i])
distance[i] = distance[u] + arr[u][i];
}
double time = t.elapsed();
cout << "Cost of the path from the initial peak to the rest:\t\n";
for (i = 0; i < c; i++) if (distance[i] != INT_MAX)
cout << m << " > " << i + 1 << " = " << distance[i] << endl;
else cout << m << " > " << i + 1 << " = " << "route not available" << endl;
cout << "Time passed: " << time << "с\n";
delete[] visited;
delete[] distance;
}
int main()
{
string filename, rash;
filename = "arr";
rash = ".dat";
int c;
cout << "Enter the number of ribs:\n";
cin >> c;
int** arr = (int**)malloc(c * sizeof(int*));
for (int i = 0; i < c; i++) {
arr[i] = (int*)malloc(c * sizeof(int));
}
filename += to_string(c) + rash;
read(arr, c, filename);
Dijkstra(arr, c, 0);
delete[] arr;
}