#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int NMAX = 100005;
long double x[NMAX],y[NMAX];
long double A;

long double arie(long double x1,long double y1,long double x2,long double y2,long double x3,long double y3)
{
    return(x2*y3+x3*y1+x1*y2-x1*y3-x2*y1-x3*y2);
}

int main()
{
    int n;
    fin >> n;
    for(int i=1;i<=n;i++) fin >> x[i] >> y[i];
    for(int i=2;i<=n-1;i++){
        A+=arie(x[1],y[1],x[i],y[i],x[i+1],y[i+1]);
    }
    A=(double)1LL*A*0.5;
    fout << setprecision(6) << fixed << abs(A);
    return 0;
}