x=[1]
y=[1]
ans=0
n=0
n=input()
for i in range(1,n+1):
x.append(0)
y.append(0)
x[i],y[i] = [int(j) for j in raw_input().split()]
for i in range(2,n+1):
ans=ans+(float)(x[i]*y[i-1]-x[i-1]*y[i])/2.0
ans=ans+(float)(x[1]*y[n]-x[n]*y[1])/2.0
if ans<0:
ans=-ans
print ans