#include<iomanip>
#include<fstream>
#include<iostream>
#include<math.h>
 
using namespace std;
 
int main(){
    ifstream in("aria.in");
    ofstream out("aria.out");
     
    int n; 
    long double xPrev, yPrev, x, y, xF, yF, sol=0;
    in>>n>>xPrev>>yPrev; 
    xF=xPrev; yF=yPrev;
     
    for(int i=1;i<=n-1;i++){
        in>>x>>y;
        sol+=(xPrev*y-x*yPrev);
        xPrev=x; yPrev=y;
    }
    sol+=(xPrev*yF-xF*yPrev);
    sol=sol/2.0;
    out<<fixed<<setprecision(5)<<fabs(sol);
    in.close(); out.close();
    return 0;
}