#include <bits/stdc++.h>

using namespace std;
struct punct
{
    double x, y;
}a[100010];
int n;
double aria;
int main()
{
    FILE *f=fopen("aria.in","r");
    fscanf(f,"%d",&n);
    for(int i=1; i<=n; i++)
        fscanf(f,"%lf%lf", &a[i].x, &a[i].y);
    for(int i=1; i<n; i++)
        aria+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    aria+=a[n].x*a[1].y-a[1].x*a[n].y;
    aria/=2;
    FILE *g=fopen("aria.out","w");
    fprintf(g,"%f",aria);
    return 0;
}