#include <cstdio>
#define MAXN 100000
double x[MAXN],y[MAXN];
inline double aria(double x1,double y1,double x2,double y2,double x3,double y3){
    return (x1*y2+x2*y3+x3*y1-y2*x3-y3*x1-y1*x2)/2;
}
int main(){
    FILE*fi,*fout;
    int i,n;
    double sum,x1,y1;
    fi=fopen("elmaj.in" ,"r");
    fout=fopen("elmaj.out" ,"w");
    fscanf(fi,"%d" ,&n);
    for(i=0;i<n;i++)
       fscanf(fi,"%lf%lf" ,&x[i],&y[i]);
    x1=x[0];
    y1=y[0];
    sum=0;
    for(i=0;i<n;i++)
      sum=sum+aria(x[i],y[i],x[(i+1)%n],y[(i+1)%n],x1,y1);
    fprintf(fout,"%lf" ,sum);
    fclose(fi);
    fclose(fout);
    return 0;
}