#include <stdio.h>
#include <stdlib.h>
float xi,yi,x1,y1,x2,y2,N,A;
int i;
int main()
{
    FILE *Fin,*Fout;
    Fin=fopen("aria.in","r");
    Fout=fopen("aria.out","w");
    fscanf(Fin,"%f",&N);
    fscanf(Fin,"%f",&xi);
    fscanf(Fin,"%f",&yi);
    x1=xi;
    y1=yi;
    for(i=1;i<N;i++)
    {
        fscanf(Fin,"%f",&x2);
        fscanf(Fin,"%f",&y2);
        A=A+x1*y2-x2*y1;
        x1=x2;
        y1=y2;
    }
    A=(A+x1*yi-xi*y1)/2;
    if(A<0)
        A=A-2*A;
    fprintf(Fout,"%.5f",A);
    fclose(Fin);
    fclose(Fout);
    return 0;
}