#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;
ifstream f1("aria.in");
ofstream f2("aria.out");

long double n,x1,y1,x2,y2,a,x,y;

int main() {
    f1>>n;
    f1>>x1>>y1;
    x=x1;
    y=y1;
    for(int i=0;i<n-1;i++){
        f1>>x2>>y2;
        a += (y1+y2)*(x2-x1)/2;
        x1=x2;
        y1=y2;
    }
    a += (y+y1)*(x-x1)/2;
    if(a < 0)
        a = 0-a;
    f2<<std::fixed<<std::setprecision(5)<<a;
    return 0;
}