#include <cstdlib>
#include <iostream>
int main(){
	double x0,y0,x1,y1,x, y, s = 0;
	char c;
	std::cin >> x0 >> c >> y0;
	x1 = x0; y1 = y0;
	while(std::cin>>x>>c>>y){
		s += x1*y - y1*x;
		x1 = x;
		y1 = y;
	}
	s += x1*y0 - y1*x0;
	std::cout<<abs(s/2)<<"\n";
}