#include <cstdio>
FILE *f=fopen("aria.in","r"),*g=fopen("aria.out","w");
using namespace std;
int n;
long double aria,vf0x,vf0y,vf1x,vf1y,vf2x,vf2y;
int main()
{
    fscanf(f,"%d%Lf%Lf%Lf%Lf",&n,&vf0x,&vf0y,&vf1x,&vf1y);
    for(n-=2;n;n--)
    {
        fscanf(f,"%Lf%Lf",&vf2x,&vf2y);
        aria+=vf0x*vf1y+vf1x*vf2y+vf2x*vf0y-vf0y*vf1x-vf1y*vf2x-vf2y*vf0x;
        vf1x=vf2x;vf1y=vf2y;
    }
    if(aria<0)aria=-aria;fprintf(g,"%Lf",aria/2.0);
    return 0;
}