#include <cstdio>

using namespace std;

struct punct
{
    double x,y;
}v[100010];

double det(punct a,punct b,punct c)
{
    return (a.x-b.x)*(a.y-c.y)-(a.x-c.x)*(a.y-b.y);
}

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    double sol=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%lf%lf",&v[i].x,&v[i].y);
    v[n+1]=v[1];
    for(int i=1;i<=n;i++) sol+=det({0,0},v[i],v[i+1]);
    printf("%f",sol/2);
    return 0;
}