unittest
{
	assert( [ "3 2", "2 1" ].parse.expand.solve == 1 );
	assert( [ "5 5", "2 3" ].parse.expand.solve == 6 );
	assert( [ "2 4", "2 4" ].parse.expand.solve == 0 );
}



import std.conv;
import std.range;
import std.stdio;
import std.typecons;

void main()
{
	stdin.byLineCopy.parse.expand.solve.writeln;
}

auto parse( Range )( Range input )
if( isInputRange!Range && is( ElementType!Range == string ) )
{
	auto HW = input.front.split.to!( long[] );
	input.popFront;
	auto hw = input.front.split.to!( long[] );
	return tuple( HW[ 0 ], HW[ 1 ], hw[ 0 ], hw[ 1 ] );
}

auto solve( long H, long W, long h, long w )
{
	return ( H - h ) * ( W - w );
}
