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Cod sursa(job #936511)
#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int main()
{
int n;
in>>n;
int x1,x2,y1,y2;
in>>x1>>y1;
double carea,totalarea=0;
for(int i=2;i<=n;++i){
in>>x2>>y2;
carea=((abs(y1)+abs(y2))*(x2-x1))/2;
totalarea+=carea;
}
out<<std::fixed<<std::setprecision(5)<<totalarea;
return 0;
}