#include <iomanip>
#include <iostream>

typedef struct {
    int x;
    int y;
} point;

double calc_s(point p[], int length);
double calc_s_triangle(point p1, point p2, point p3);
double calc_s_triangle(point p1, point p2);

int main()
{
    point p[100];
    int number_point;
    int px,py;
    std::cin >> number_point;
    for (int i = 0; i < number_point; i++) {
        scanf("%d %d", &px, &py);
        p[i].x = px;
        p[i].y = py;
    }
    std::cout <<std::fixed << std::setprecision(1) << calc_s(p, number_point) << std::endl;
    return 0;
}

double calc_s(point p[], int length)
{
    double area = 0;
    for (int i = 1; i < length - 1; i++) {
        area += calc_s_triangle(p[0], p[i], p[i+1]);
    }
    return area;
}

double calc_s_triangle(point p1, point p2, point p3)
{
    //p3???????????????????§????
    p1.x -= p3.x;
    p1.y -= p3.y;
    p2.x -= p3.x;
    p2.y -= p3.y;
    return calc_s_triangle(p1, p2);
}

double calc_s_triangle(point p1, point p2) {
    double s = (p1.x * p2.y - p1.y * p2.x) / 2.0;
    return s;
}