#include <bits/stdc++.h>
using namespace std;

const int PMAX = 100005;

struct POINT {
    double x, y;
};
POINT v[PMAX];

int main(void){
    FILE *fi = fopen("aria.in","r");
    FILE *fo = fopen("aria.out","w");
    int n;
    int i;
    double area;

    area = 0.;

    fscanf(fi,"%d",&n);
    for(i=0; i<n; ++i)
        fscanf(fi,"%lf%lf",&v[i].x,&v[i].y);
    for(i=1; i<n; ++i)
        area+=(v[i].x-v[i-1].x)*(v[i].y+v[i-1].y);
    area+=(v[0].x-v[n-1].x)*(v[0].y+v[n-1].y);

    area *= 0.5;

    fprintf(fo,"%f",abs(area));

    fclose(fi);
    fclose(fo);
    return 0;
}