Calculating the 2D prefix sum
for ( int i = 1 ; i <= n ; i ++ ) { for ( int j = 1 ; j <= m ; j ++ ) { prefix_2D [ i ] [ j ] += prefix_2D [ i ] [ j - 1 ] ; } } for ( int i = 1 ; i <= n ; i ++ ) { for ( int j = 1 ; j <= m ; j ++ ) { prefix_2D [ i ] [ j ] += prefix_2D [ i - 1 ] [ j ] ; } } int answer = 0 ;