#include <iostream>
#include <vector>
vector<double> X,Y;
int main(){
	double x,y,s=0;
	int i,n=0;
	char c;
	while(std::cin>>x>>c>>y){
		X.push_back(x);
		Y.push_back(y);
		n++;
	}
	X.push_back(X[0]);
	Y.push_back(Y[0]);
	for(i=0;i<n;i++)
		s+=(X[i]*Y[i+1]-X[i+1]*Y[i]);
	std::cout<<abs(s)/2<<endl;
}