#include <iostream>
#include <cmath>

int main() {
	double cx[20], cy[20];
	int i;
	double a, b, c, z;
	double S;
	char dummy;
	
	i=0;
	while( std::cin >> cx[i] >> dummy >> cy[i] ) i++; i++;

	S = 0.0;
	for( int j=0; j<i-2; j++) {
		a = sqrt( (cx[0]-cx[j+1])*(cx[0]-cx[j+1]) + (cy[0]-cy[j+1])*(cy[0]-cy[j+1]) );
		b = sqrt( (cx[0]-cx[j+2])*(cx[0]-cx[j+2]) + (cy[0]-cy[j+2])*(cy[0]-cy[j+2]) );
		c = sqrt( (cx[j+1]-cx[j+2])*(cx[j+1]-cx[j+2]) + (cy[j+1]-cy[j+2])*(cy[j+1]-cy[j+2]) );
		z = ( a + b + c ) / 2.0;
		
		S += sqrt( z*(z-a)*(z-b)*(z-c) );
	}
	
	std::cout << S << std::endl;
	return 0;