#include <bits/stdc++.h>

#define NMax 100005

using namespace std;

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

int N;
struct Punct {
    double x,y;
}punct[NMax];
double aria;

void Aria()
{
    punct[N+1]=punct[1];
    for(int i=1;i<=N;i++)
        aria+=(punct[i].x*punct[i+1].y-punct[i].y*punct[i+1].x);
    aria/=2;
}

int main()
{
    fin>>N;
    for(int i=1;i<=N;i++)
        fin>>punct[i].x>>punct[i].y;
    Aria();
    fout<<fixed<<setprecision(6)<<aria;

    return 0;
}