#include <bits/stdc++.h>

using namespace std;

ifstream inf("aria.in");
ofstream outf("aria.out");

int n;
float arie;
vector<pair<int,int>> pct;

int main()
{
    inf>>n;
    for(int i=1; i<=n; i++)
    {
        int x,y;
        inf>>x>>y;
        pct.push_back({x,y});
    }
    for(int i=0; i<n-1; i++)
        arie+=1.0*(pct[i].first*pct[i+1].second-pct[i+1].first*pct[i].second)/2;
    arie+=1.0*(pct[n-1].first*pct[0].second-pct[0].first*pct[n-1].second)/2;
    outf<<setprecision(5)<<fixed;
    outf<<arie;

    return 0;
}