C/C++ :: Converting 2D Array Code To Malloc

I made a program that adds two matrices and displays their sum with a max dimension of 100.

/* This program asks the user for 2 matrices called A and B, as integers,
and displays their sum, C. The max dimension of each matrix is 100. */

#include <stdio.h>
// Construct function
void construct() {
int m, n, i, j; // Variables
int first[100][100], second[100][100], sum[100][100]; // Matrices variables

[Code] ....

Now I need to change it so there is no max size for each matrix. The arrays will be larger than 100x100 so I need to use malloc to create my arrays. So I cant use int A[rows][cols]. This is what I did to covert arrays to malloc. It compiles but it crashes after I entered all the integers.

/* This program asks the user for 2 matrices called A and B, as integers,
and displays their sum, C. The max dimension of each matrix is 100. */

#include <stdio.h>
#include <stdlib.h>
// Construct function
void construct() {
int m, n, i, j; // Variables
int *first = NULL;

[Code] .....