#include<iostream>
#include<complex>
#include<math.h>
#include<cstdio>
using namespace std;
typedef complex<double> xy_t;
xy_t P[110];

double area(xy_t a, xy_t b) {return (conj(a)*b).imag();}

int main()
{
    int N;
    cin >> N;
    int x, y;
    for (int i = 0; i < N; i++) {
        cin >> x >> y;
        P[i] = xy_t((double)x, (double)y);
    }
    double sum = 0.0;
    xy_t a = P[0];
    for (int i = 0; i < N - 2; i++) {
        xy_t b = P[i + 1], c = P[i + 2];
        sum += area(c - a, b - a) / 2.0;
    }
    sum = abs(sum);
    printf("%.1f\n", sum);
    return 0;
}