#include<bits/stdc++.h>

using namespace std;

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

int main(){
    int n,i;
    long double x1,x2,y1,y2,firstx,firsty,A=0;
    fin>>n;
    fin>>firstx>>firsty;
    x1 = firstx;
    y1 = firsty;
    for(i = 2; i <= n; i++){
        fin>>x2>>y2;
        A += (x1/2)*y2-(y1/2)*x2;
        x1=x2;
        y1=y2;
    }
    A += (x1/2)*firsty-(y1/2)*firstx;
    if(A < 0){
        A = -A;
    }
    fout<<A<<'\n';
	return 0;
}