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#include <bits/stdc++.h>
using namespace std;
#define int long double
signed main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int n,a,b,x,y,aa,bb,ans=0;
    cin >> n >> a >> b;
    aa=a;bb=b;
    for(int i=1;i<n;i++){
        cin >> x >> y;
        ans+=a*y-x*b;
        a=x;
        b=y;
    }
    ans+=bb*-aa*y;
    ans/=2;
    ans=abs(ans);
    cout << fixed << setprecision(5)<< ans;
    return 0;
}