import sys
readline=sys.stdin.readline
write=sys.stdout.write

N=int(readline())
X,Y=[],[]
for _ in range(N):
    x,y=map(int,readline().split())
    X.append(x)
    Y.append(y)
ans=0
for i in range(N):
    ans+=X[i]*Y[(i+1)%N]-Y[i]*X[(i+1)%N]
print(ans)