#include <bits/stdc++.h>
using namespace std;
const int nmx = 1e5 + 3;
typedef valarray<long double> P;
#if 1
#define cin fin
#define cout fout
ifstream fin("aria.in");
ofstream fout("aria.out");
#endif // 0
P p[nmx];
double aria(P a, P b, P c){
P ab = b-a, ac = c-a;
return .5*(ab[0]*ac[1]-ab[1]*ac[0]);
}
int main(){
int n;
cin >> n;
for(int i=0;i<n;i++){
p[i] = {0,0};
cin >> p[i][0] >> p[i][1];
}
long double arie = 0;
for(int i=0;i<n-1;i++)
arie += aria({0,0},p[i],p[i+1]);
arie += aria({0,0},p[n-1],p[0]);
cout <<fixed<< abs(arie);
}