How do I fix this error?

I wrote a program to add two matrices. The program I wrote takes in the number of rows and columns (max columns is 50) and then adds them. The 2D arrays required are initialized dynamically. However, the debugger is giving me an error in the addition part. Here's my code:

#include <iostream>
#include <iomanip>
#include <conio.h>
#include <cstring>

using namespace std;

int main()
{
	int rows1, columns1,
	cout << "Enter the number of rows: ";
	cin >> rows1;
	cout << "Enter the number of columns (max 50): ";
	cin >> columns1;
	if(columns1 > 50)
	{
		cout << "Columns cannot be more than 50.";
		exit(0);
	}

	int* matrix1 = new int[rows1*50]; // Create a 2D array of size [rows][50]

		// Input first matrix
		for(int i = 0; i < rows1; i++)
		{
			for(int j = 0; j < columns1; j++)
			{
				cout << "Enter A[" << i + 1 << "][" << j + 1 << "]: ";
				cin >> matrix1[i*50 + j]; // For 2D arrays, a[i][j] is a[i*columns + j]
			}
		}

		// Show matrix A to user
		for(int i = 0; i < rows1; ++i)
		{
			cout << endl;
			for(int j = 0; j < columns1; ++j)
				cout << "  " << matrix1[i*50 + j];
		}

		cout << endl << endl;

		// Input second matrix
		int* matrix2 = new int[rows1*50];

		for(int i = 0; i < rows1; i++)
		{
			for(int j = 0; j < columns1; j++)
			{
				cout << "Enter B[" << i + 1 << "][" << j + 1 << "]: ";
				cin >> matrix2[i*50 + j]; 
			}
		}

		// Show matrix B to user
		for(int i = 0; i < rows1; ++i)
		{
			cout << endl;
			for(int j = 0; j < columns1; ++j)
				cout << "  " << matrix2[i*50 + j];
		}

		// Make a new matrix of same size to hold the finished array
		int* matrix3 = new int[rows1*50];
		for(int i = 0; i < rows1; ++i)
			{
				for(int j = 0; j < columns1; ++j)
					matrix3[i*50 + j] = matrix1[i*50 + j] + matrix2[i*50 + j];
			}
		cout << "The result is: \n";
		// Show resultant matrix to user
		for(int i = 0; i < rows1; ++i)
		{
			cout << endl;
			for(int j = 0; j < columns1; ++j)
				cout << "  " << matrix3[i*50 + j];
		}
		delete [] matrix1;
		delete [] matrix2;
		delete [] matrix3;

		matrix1 = nullptr;
		matrix2 = nullptr;
		matrix3 = nullptr;
	_getch();
	return 0;
}

I'm getting the error in the second last for loop - where the actual adding takes place

Thanks,

Rahul