x1,y1,x2,y2=map(int,input().split())
c=0
l=((y2-y1)//2)+1
if (y2-y1)%2==0:
t=(y2-y1)//2
else:
t=(y2-y1+1)//2
big=(x2-x1)//2+1
if (x2-x1)%2!=0: small=((x2-x1)+1)//2
else: small=(x2-x1)//2
print(t*small+l*big)1733A - Consecutive Sum | 1733B - Rule of League |
1733C - Parity Shuffle Sorting | 1264A - Beautiful Regional Contest |
1695A - Subrectangle Guess | 467B - Fedor and New Game |
252C - Points on Line | 735C - Tennis Championship |
992A - Nastya and an Array | 554A - Kyoya and Photobooks |
79B - Colorful Field | 265B - Roadside Trees (Simplified Edition) |
1362C - Johnny and Another Rating Drop | 1214C - Bad Sequence |
1091B - New Year and the Treasure Geolocation | 244A - Dividing Orange |
1061C - Multiplicity | 1312A - Two Regular Polygons |
801A - Vicious Keyboard | 510B - Fox And Two Dots |
616D - Longest k-Good Segment | 1604A - Era |
555B - Case of Fugitive | 551A - GukiZ and Contest |
1399F - Yet Another Segments Subset | 1371C - A Cookie for You |
430B - Balls Game | 1263A - Sweet Problem |
1332B - Composite Coloring | 254A - Cards with Numbers |