def main():
    n = int(input())
    xx, yy = [], []
    for i in range(n):
        x, y = map(int, input().split())
        if i > 0:
            x -= xx[0]
            y -= yy[0]
        xx.append(x)
        yy.append(y)

    ans = 0
    for i in range(1, n - 1):
        ans += abs(xx[i] * yy[i + 1] - yy[i] * xx[i + 1])
    print(ans)


if __name__ == "__main__":
    main()