#include <bits/stdc++.h>

using namespace std;

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

const int NMax = 1e5 + 5;

pair < int, int > A[NMax];

int main(){
    int n, x, y;
    long double aria = 0;
    fin >> n;
    for(int i = 1; i <= n; i++){
        fin >> x >> y;
        A[i] = {x, y};
    }
    A[n + 1] = A[1];
    for(int i = 2; i <= n + 1; i++){
        aria += (1LL * A[i].second * A[i - 1].first - 1LL * A[i].first * A[i - 1].second);
    }
    aria /= 2.0f;
    fout << fixed << setprecision(6) << aria;
    return 0;
}