#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int n;
struct punct
{
    double a;
    double b;
}s[100001];
void citire()
{
    scanf("%d", &n);
    for(int i=0;i<n;i++)
        {
            scanf("%lf %lf", &s[i].a , &s[i].b);
        }
}
double p;
double arieP;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    citire();
    for(int i=0;i<n-1;i++)
    {
        p=0.5*((s[i].a*s[i+1].b)-(s[i+1].a*s[i].b));
        arieP+=p;
    }
    p=0.5*((s[n-1].a*s[0].b)-(s[0].a*s[n-1].b));
    arieP+=p;
    printf("%lf", arieP);
    return 0;
}