#include <bits/stdc++.h>
#define int long double
#define cin in
#define cout out

using namespace std;

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

int segmentaria(pair<int,int>A,pair<int,int>B,pair<int,int>C)
{
    return A.first*B.second+B.first*C.second+C.first*A.second-
    A.second*B.first-B.second*C.first-C.second*A.first;
}

int segment(pair<int,int>A,pair<int,int>B) {
    return segmentaria({0,0},A,B);
}

int32_t main() {

    int32_t n;
    cin>>n;


    vector<pair<int,int>>v(n+2,{0,0});
    for (int32_t i=1;i<=n;i++) {
        cin>>v[i].first>>v[i].second;
    }

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

    int arie=0;
    for(int32_t i=1;i<=n;i++)
        arie+=segment(v[i],v[i+1]);

    cout<<abs(arie)/2<<endl;
    return 0;
}