#include <stdio.h>
#include <stdlib.h>
int main(){
    FILE *fin=fopen("aria.in","r");
    FILE *fout=fopen("aria.out","w");
    int n,i;
    double a,xa,ya,xp,yp,x,y;

    fscanf(fin,"%d",&n);
    fscanf(fin,"%lf%lf",&xp,&yp);
    a=0;
    xa=xp;
    ya=yp;
    for(i=1; i<n; i++){
        fscanf(fin,"%lf%lf",&x,&y);
        a+=(xa-x)*(ya+y);
        xa=x;
        ya=y;
    }
    a+=(x-xp)*(y+yp);
    fprintf(fout,"%lf\n",fabs(a/2));
    fclose(fin);
    fclose(fout);
    return 0;
}