#include <cstdio>



using namespace std;



int n;



struct pct

{

    double x, y;

}point[100001];



double aria=0;



void citire(int n)

{

    for(int i=0; i<n; i++)

        scanf("%lf %lf", &point[i].x, &point[i].y);

}



int main()

{

    freopen("aria.in", "r", stdin);

    freopen("aria.out", "w", stdout);

    scanf("%d", &n);

    citire(n);

    for(int i=0; i<n-1; i++)

    {

        double ariaPrim= 0.5*((point[i].x*point[i+1].y)-(point[i+1].x*point[i].y));

        aria+=ariaPrim;

    }

    double ariaPrim= 0.5*((point[n-1].x*point[0].y)-(point[0].x*point[n-1].y));

    aria+=ariaPrim;

    printf("%lf", aria);

    return 0;