#include<algorithm>
#include<bitset>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<deque>
#include<fstream>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<vector>

using namespace std;

#ifdef HOME
const string inputFile = "input.txt";
const string outputFile = "output.txt";
#else
const string problemName = "aria";
const string inputFile = problemName + ".in";
const string outputFile = problemName + ".out";
#endif

const int NMAX = 100000 + 5;

int N;
pair<double, double> P[NMAX];
double sum;

int main() {
    int i;

#ifndef ONLINE_JUDGE
    freopen(inputFile.c_str(), "r", stdin);
    freopen(outputFile.c_str(), "w", stdout);
#endif

    scanf("%d", &N);

    for(i = 1; i <= N; i++)
        scanf("%lf%lf", &P[i].first, &P[i].second);

    P[N + 1] = P[1];

    for(i = 1; i <= N; i++)
        sum += (P[i].first * P[i + 1].second - P[i + 1].first * P[i].second);

    printf("%.10f\n", fabs(sum / 2.0));

    return 0;
}