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*<sub class="lower-index">1</sub>, *y*<sub class="lower-index">1</sub>) and (*x*<sub class="lower-index">2</sub>, *y*<sub class="lower-index">2</sub>), where *x*<sub class="lower-index">1</sub> ≤ *x*<sub class="lower-index">2</sub> and *y*<sub class="lower-index">1</sub> ≤ *y*<sub class="lower-index">2</sub>, then all cells having center coordinates (*x*, *y*) such that *x*<sub class="lower-index">1</sub> ≤ *x* ≤ *x*<sub class="lower-index">2</sub> and *y*<sub class="lower-index">1</sub> ≤ *y* ≤ *y*<sub class="lower-index">2</sub> will be filled. Orthogonal coordinates system is set up so that one of cell sides is parallel to *OX* axis, all hexagon centers have integer coordinates and for each integer *x* there are cells having center with such *x* coordinate and for each integer *y* there are cells having center with such *y* coordinate. It is guaranteed that difference *x*<sub class="lower-index">2</sub> - *x*<sub class="lower-index">1</sub> is divisible by 2.

Working on the problem Petya decided that before painting selected units he wants to output number of units that will be painted on the map.

Help him implement counting of these units before painting.
The only line of input contains four integers *x*<sub class="lower-index">1</sub>, *y*<sub class="lower-index">1</sub>, *x*<sub class="lower-index">2</sub>, *y*<sub class="lower-index">2</sub> ( - 10<sup class="upper-index">9</sup> ≤ *x*<sub class="lower-index">1</sub> ≤ *x*<sub class="lower-index">2</sub> ≤ 10<sup class="upper-index">9</sup>,  - 10<sup class="upper-index">9</sup> ≤ *y*<sub class="lower-index">1</sub> ≤ *y*<sub class="lower-index">2</sub> ≤ 10<sup class="upper-index">9</sup>) — the coordinates of the centers of two cells.
none
1 1 5 5
13
-1 -3 1 3
11
-2 -2 2 2
13
0 0 2 2
5
0 0 2 0
2
0 0 0 0
1
0 -2 0 2
3
-2 -2 -2 0
2
-1000000000 -1000000000 1000000000 1000000000
2000000002000000001
-999999999 -999999999 999999999 999999999
1999999998000000001
-999999999 -999999999 -1 -1
499999999000000001
-411495869 33834653 -234317741 925065545
78953311064369599
-946749893 -687257665 -539044455 -443568671
49676664342971903
-471257905 -685885153 782342299 909511043
999994499807710193
-26644507 -867720841 975594569 264730225
567493356068872580
-537640548 -254017710 62355638 588691834
252811256874252458
309857887 -687373065 663986893 403321751
193123336242128360
-482406510 -512306894 412844236 -168036050
154104365578285608
-330513944 -970064382 500608496 369852884
556817654843544374
-157778763 218978791 976692563 591093087
211076500156631060
1000000000 1000000000 1000000000 1000000000
1
1 0 5 6
18
-1 -4 1 4
14
-2 -3 2 3
18
0 -1 2 3
8
0 -1 2 1
5
0 -1 0 1
2
0 -3 0 3
4
-2 -3 -2 1
3
-1000000000 -999999999 1000000000 999999999
2000000000000000000
-999999999 -1000000000 999999999 1000000000
2000000000000000000
-999999999 -1000000000 -1 0
500000000000000000
-411495869 33834652 -234317741 925065546
78953311241547728
-946749893 -687257666 -539044455 -443568670
49676664750677342
-471257905 -685885154 782342299 909511044
999994501061310398
-26644507 -867720842 975594569 264730226
567493357071111657
-537640548 -254017711 62355638 588691835
252811257474248645
309857887 -687373066 663986893 403321752
193123336596257367
-482406510 -512306895 412844236 -168036049
154104366473536355
-330513944 -970064383 500608496 369852885
556817655674666815
-157778763 218978790 976692563 591093088
211076501291102387
1000000000 999999999 1000000000 999999999
1
x1,y1,x2,y2 = map(int,input().split())
print((x2-x1)//2*(y2-y1+1)+(y2-x1)//2-(y1-x1)//2+1)