#include <bits/stdc++.h>

#define point   std::pair <int, int>
#define llg     long long

int N;
std::vector <point> V;
llg surface(std::vector <point> &V) {
    llg sum = 0;
    int N = V.size();
    for (int i=0; i<N; ++i)
        sum = (sum + 1ll*V[i].first*V[(i+1)%N].second - 1ll*V[(i+1)%N].first*V[i].second);
    return sum;
}

#define FILENAME    std::string("aria")
std::ifstream input (FILENAME+".in");
std::ofstream output(FILENAME+".out");

int32_t main()
{
    input >> N;
    V.resize(N);
    for (auto &it:V) input >> it.first >> it.second;
    output << (long double)surface(V)/2;

    return 0;
}