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Cod sursa(job #963648)
#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double x[100005],y[100005],arie=0.0;
int n;
int main()
{
f>>n;
for (int i=1; i<=n; i++) f>>x[i]>>y[i];
x[n+1]=x[1]; y[n+1]=y[1];
for (int i=1; i<=n; i++) arie+=(x[i]*y[i+1]-y[i]*x[i+1]);
arie*=0.5;
g<<fixed<<setprecision(5)<<fabs(arie)<<'\n';
}