C++ code terminates by Segmentation Fault?

#include <iostream>

using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    int num, qr, maxSize = 0;
    cin >> num >> qr;
    for (int i = 0; i < num; i++)
    {
        int size;
        cin >> size;
        if (size > maxSize)
        {
            maxSize = size;
        }
    }
    int arr[num][maxSize];
    for (int i = 0; i < num; i++)
    {
        int s;
        cin >> s;
        for (int j = 0; j < s; j++)
        {
            int elem;
            cin >> elem;
            arr[i][j] = elem;
        }
    }
    for (int i = 0; i < qr; i++)
    {
        int a,b;
        cin >> a >> b;
        cout << arr[a][b];
    }
    return 0;
}

This code terminates giving Segmentation Fault Error. I am new to the world of C++ and I don't have any idea of this error message. Any help would mean a lot to me.