#include<cstdio>
using namespace std;
FILE *f=fopen("aria.in","r");
FILE *g=fopen("aria.out","w");
double x1,y1,x2,y2,arie;
int main()
{
    int i,n;
    fscanf(f,"%d",&n);
    fscanf(f,"%lf%lf",&x1,&y1);
    for(i=2;i<=n;i++)
    {
        fscanf(f,"%lf%lf",&x2,&y2);
        arie+=0.5*(y1+y2)*(x2-x1);
        x1=x2;
        y1=y2;
    }
    if(arie<0)arie=(-1)*arie;
    fprintf(g,"%.5lf",arie);
    return 0;
}