#include <iostream>
#include<cstdio>
#include<fstream>
#include<iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double v[2][100001];
int main()
{
  //eliza copiaza
    int n,i;
 //  FILE*fin,*fout;
    double s=0.0;
//    fin=fopen("aria.in","r");
//    fout=fopen("aria.out","w");
    fin>>n>>v[0][1]>>v[1][1];
    for(i=2;i<=n;i++){
      fin>>v[0][i]>>v[1][i];
      s+=(v[0][i-1]-v[0][i])*(v[1][i-1]+v[1][i]);
    }
    s+=(v[0][n]-v[0][1])*(v[1][n]+v[1][1]);
    s*=0.5;
    fout<<fixed<<setprecision(6)<<s;
    return 0;
}