#include <iostream>
#include <fstream>
#include <cmath>
#include <vector>

using namespace std;

void trap(int n, vector <double> x, vector <double> y, double &sum, int nd)
{
      	sum=0.e0;
	for(int i=1;i<n;i++)	
	{
        	if(x[i]<x[i-1])
		{
		  cout<<"x NOT in INCREASING order"<<endl;
		  break;
		}
        	sum=sum+.5e0*(x[i]-x[i-1])*(y[i]+y[i-1]);
      	}
      	return;
}

int main(int argc, char** argv)
{
	string line;
      	int n=0,nd=1000;
      	vector <double> x(nd),y(nd);

      	ifstream in("points.dat");
      	for(int i=0;i<nd;i++)
	{
		in>>x[i]>>y[i];
		getline(in,line);
		if(in.eof())
		{
			n = i;
			break;
		}
	}
      	in.close();
	double sum = 0.e0;
      	trap(n,x,y,sum,nd);
 	cout<<"Sum = "<<sum<<endl;
	return 0;
}