lucky square c++ help??

[9i8XizwU]

I'm trying to create a lucky square. A lucky sequence means that every number after the first two is the sum of the two preceding number. A lucky square means that all rows and columns are a lucky sequence.
for example:

input size of a square:
4
input numbers row by row:
2 4 6
3 5 8
5 9 14

row: Y Y Y
cols: Y Y Y

This is a lucky square.

I have tried up till here, how could I change the codes so that the sum of two numbers results in the number after?

#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
const size_t M = 10;
int a[M][M] = {0};
cout << "Please input the size of square: ";
size_t m;
cin >> m;
if (!(m < M) || (m == 0)) m = M - 1;
cout << endl;
cout << "Please input the square row by row: ";
for (size_t i = 0; i < m; i++)
{
for (size_t j = 0; j < m; j++) cin >> a[i][j];
}
for (size_t i = 0; i < m; i++)
{
for (size_t j = 0; j < m; j++)
{
a[i][m] += a[i][j];
a[m][j] += a[i][j];
}
}
cout << endl;
for (size_t i = 0; i < m + 1; i++)
{
for (size_t j = 0; j < m + 1; j++)
{
cout << setw(3) << a[i][j] << ' ';
}
cout << '\n';
}
cout << endl;
}

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