why this program is not printing the correct second element of returned iterator?

I want to change the value of the map element if it is already exist in the map container i.e count the no of the elements in that container

std :: map < int, int > m;
std :: map < int, int > :: iterator itr;

int arr[] = { 10, 40, 20, 20, 20, 20, 20, 20, 10, 30, 10, 30, 40 };

for (int i : arr) {
    itr = m.find(i);
    if (itr == m.end() ) {
        int value = 0;
        m.insert(std :: make_pair(i, value));
    } else {
        ++itr->second;
    }
}
itr = m.begin();
while (itr != m.end() ) {
    std :: cout << itr->first << " -> " << itr->second << std :: endl;
    ++itr;
}

I got wrong output :

10 -> 2
20 -> 5
30 -> 1
40 -> 1