#include <stdio.h>
int main(){
    int n, i;
    double x1, y1, x2, y2, arie, a, b;
    FILE *fin, *fout;
    fin=fopen("aria.in", "r");
    fout=fopen("aria.out", "w");
    fscanf(fin, "%d%lf%lf", &n, &x1, &y1);
    a=x1;
    b=y1;
    arie=0;
    for(i=1; i<n; i++){
        fscanf(fin, "%lf%lf", &x2, &y2);
        arie+=x1*y2-x2*y1;
        x1=x2;
        y1=y2;
    }
    arie+=x1*b-a*y1;
    arie*=0.5;
    if(arie<0){
        arie=-arie;
    }
    fprintf(fout, "%.5lf", arie);
    fclose(fin);
    fclose(fout);
    return 0;
}