A.Sredina na kolona

#include <iostream>

using namespace std;

int main()
{
    int n, m, c;
    cin >> n >> m;

    // sozdavash matrica 'a' so 'n' po 'm' elementi
    int a[n][m] = {};

    // gi vnesuvash elementite
    for (int i = 0; i < n; i++) {
        for (int k = 0; k < m; k++) {
            cin >> a[i][k];
        }
    }

    // samo tuka menuvame...
    // gi sobirame site elementi od edna redica

    cout << "Vnesi kolona : ";
    cin >> c;

    float zbir = 0;
    for (int i = 0; i < n; i++) { // tuka sega 'n'
        zbir += a[i][c]; // sega [i][c] mesto [r][i]
    }

    cout << "Aritmetichka sredina : " << zbir / (n) << endl;

    return 0;
}