#include <cstdio>
#include <iostream>
#define f first
#define s second
using namespace std;

const int nmx = 100004;

int n,i;
double S=0;
pair <double,double> v[nmx];

int main()
{

    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d",&n);
    scanf("%lf %lf",&v[1].f,&v[1].s);
    for(i=2; i<=n; i++)
    {
        scanf("%lf %lf",&v[i].f,&v[i].s);
        S += (v[i-1].f-v[i].f)*(v[i].s+v[i-1].s);
    }
    S += (v[n].f-v[1].f)*(v[n].s+v[1].s);
    S /= 2;
    printf("%.5lf",S);
    return 0;
}