#include<cstdio>
#include<cmath>
using namespace std;
struct pct{
    double x;
    double y;
}v[100100];
int n,i,j;
double s,ss;
FILE *f,*g;
int main(){
    f=fopen("aria.in","r");
    g=fopen("aria.out","w");
    fscanf(f,"%d",&n);
    for(i=1;i<=n;i++){
        fscanf(f,"%lf%lf",&v[i].x,&v[i].y);
    }
    v[n+1]=v[1];
    for(i=1;i<=n;i++){
        s = s + ( v[i].x * v[i+1].y - v[i+1].x * v[i].y ) / 2;
    }
    fprintf(g,"%.5lf",s);

    fclose(f);
    fclose(g);
    return 0;
}