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

int n;
const int maxN=(1e5)+5;

pair<long double,long double> v[maxN];

long double S;

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

    fin>>n;

    for(int i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;

    v[n+1]=v[1];

    for(int i=1;i<=n;i++)
        S+=(v[i].first*v[i+1].second-v[i+1].first*v[i].second);

    S*=0.5;

    if(S<0.0) S=-S;

    fout<<setprecision(5)<<fixed<<S<<'\n';

    return 0;
}