I get this error when submitting a solution:
PHP Notice(8): Undefined offset: -2 in core/inc/3p/EvalMath.php line 384
Backtrace starts in www/challenge/training/math/pyramid/index.php line 42.
math_pyramid_check()...... www/challenge/training/math/pyramid/index.php line 73.
EvalMath->evaluate()...... core/inc/3p/EvalMath.php line 133.
EvalMath->nfx()........... core/inc/3p/EvalMath.php line 216.
EvalMathStack->last()..... core/inc/3p/EvalMath.php line 384.
GWF_Debug::error_handler() core/inc/util/GWF_Debug.php line 183.