#include <bits/stdc++.h>
#define double long double
#define Point Vector
usingnamespacestd;structVector{doublex,y;Vector(){x=y=0;}Vector(doublexx,doubleyy){x=xx;y=yy;}Vectoroperator-(constVector&v)const{returnVector(x-v.x,y-v.y);}doubleoperator%(constVector&v)const{returnx*v.y-y*v.x;}friendistream&operator>>(istream&is,Vector&v){is>>v.x>>v.y;returnis;}};intn;vector<Point>p;doublepolygonArea(constvector<Point>&polygon){doublearea=polygon.back()%polygon.front();for(inti=0;i+1<polygon.size();++i){area+=polygon[i]%polygon[i+1];}returnfabs(area)/2;}intmain(){cin.tie(nullptr)->sync_with_stdio(false);cin>>n;p.resize(n);for(inti=0;i<n;++i){cin>>p[i];}cout<<setprecision(6)<<fixed;cout<<polygonArea(p);return0;}