N = int(input())
X=[];Y=[]
ans = 0
for _ in range(N):
    x,y = map(int,input().split())
    X.append(x); Y.append(y)
for i in range(1,N-1):
    p,q = X[i] - X[0], Y[i] - Y[0]
    r,s = X[i+1] - X[0], Y[i+1] - Y[0]
    ans += abs(p*s - r*q)
print(ans)