fix BlockGasLimit verification

libethcore/BlockInfo.cpp

@@ -246,9 +246,9 @@ void BlockInfo::verifyParent(BlockInfo const& _parent) const
 		BOOST_THROW_EXCEPTION(InvalidDifficulty() << RequirementError((bigint)calculateDifficulty(_parent), (bigint)difficulty));
 
 	if (gasLimit < c_minGasLimit ||
-		gasLimit < _parent.gasLimit * (c_gasLimitBoundDivisor - 1) / c_gasLimitBoundDivisor ||
-		gasLimit > _parent.gasLimit * (c_gasLimitBoundDivisor + 1) / c_gasLimitBoundDivisor)
-		BOOST_THROW_EXCEPTION(InvalidGasLimit() << errinfo_min((bigint)_parent.gasLimit * (c_gasLimitBoundDivisor - 1) / c_gasLimitBoundDivisor) << errinfo_got((bigint)gasLimit) << errinfo_max((bigint)_parent.gasLimit * (c_gasLimitBoundDivisor + 1) / c_gasLimitBoundDivisor));
+		gasLimit < _parent.gasLimit - _parent.gasLimit / c_gasLimitBoundDivisor ||
+		gasLimit > _parent.gasLimit + _parent.gasLimit / c_gasLimitBoundDivisor)
+		BOOST_THROW_EXCEPTION(InvalidGasLimit() << errinfo_min((bigint)_parent.gasLimit - _parent.gasLimit / c_gasLimitBoundDivisor) << errinfo_got((bigint)gasLimit) << errinfo_max((bigint)_parent.gasLimit + _parent.gasLimit / c_gasLimitBoundDivisor));
 
 	// Check timestamp is after previous timestamp.
 	if (parentHash)