nested loops to generate all 6 digit numbers
for ( int i = 0 ; i < 10 ; ++ i ) { for ( int j = 0 ; j < 10 ; ++ j ) { for ( int k = 0 ; k < 10 ; ++ k ) { for ( int l = 0 ; l < 10 ; ++ l ) { for ( int m = 0 ; m < 10 ; ++ m ) { for ( int n = 0 ; n < 10 ; ++ n ) { if ( i + j + k == l + m + n ) {