CF630E A rectangle

Developing tools for creation of locations maps for turn-based fights in a new game, Petya faced the following problem. A field map consists of hexagonal cells. Since locations sizes are going to be big, a game designer wants to have a tool for quick filling of a field part with identical enemy units. This action will look like following: a game designer will select a rectangular area on the map, and each cell whose center belongs to the selected rectangle will be filled with the enemy unit. More formally, if a game designer selected cells having coordinates $ (x_{1},y_{1}) $ and $ (x_{2},y_{2}) $ , where $ x_{1}

The only line of input contains four integers $ x_{1},y_{1},x_{2},y_{2} $ ( $ -10^{9}

Output one integer — the number of cells to be filled.