#include<bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");ofstream fout("aria.out");
int main(){
long int n;fin>>n;
long double a,b,x1,x2,y1,y2;long double aria=0;fin>>x1>>y1;a=x1;b=y1;
for(int a=1;a<n;a++){
    fin>>x2>>y2;
    aria+=x1*y2-x2*y1;
    x1=x2;y1=y2;
}
aria+=x1*b-a*y1;aria/=2;if(aria<0)aria*=-1;fout<<fixed<<setprecision(6)<<aria;

return 0;
}