#include <cstdio>
#include <bits/stdc++.h>
using namespace std;
int n;
long double aria,x0,y0,x1,y1,x2,y2;
int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin>>n;
    fin>>x0>>y0;
    fin>>x1>>y1;
    for(n-=2;n;n--)
    {
        fin>>x2>>y2;
        aria+=x0*y1+x1*y2+x2*y0-y0*x1-y1*x2-y2*x0;
        x1=x2;y1=y2;
    }
    if(aria<0)aria=-aria;
    fout<<aria/2.0;
    return 0;
}