#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Yukicoder №1381 Simple Geometry 1
// https://yukicoder.me/problems/no/1381
// □ABCD = |AB| * |BC| = X
// △ABP = ( |AB| * |AP| )/2
// △BCZ = ( |BC| * |CQ| ) /2
// △DPQ = ( |DP| * |DQ| ) /2
// △BPQ = □ABCD - △ABP - △BCZ - △DPQ
// 	 = X - (|AB|*Y)/2 - (|BC|*Z)/2 -((|AB|-Y)*(|CD|-Z))/2
//	 = X - ( (|AB|*|BC|)+(Y * Z) ) / 2
//	 = X - ( X +(Y * Z) ) / 2
//	 = ( X - (Y * Z) ) / 2
// |AP| = Y
// |CQ| = Z
// |BP| = |BQ| + W
// W = |BP| - |BQ|
// ***********************
// for debug
#define DEBUG

#define NOP do{}while(0)
#ifdef DEBUG
#define TRACE(...) do{printf(__VA_ARGS__);fflush(stdout);}while(0)
#define TRACECR do{printf("\n");fflush(stdout);}while(0)
#else
#define TRACE(...) NOP
#define TRACECR NOP
#endif

#define PRINCR printf("\n")
#define NOCR(strig) do{char *p;p=strchr(strig,'\n');if(p)*p='\0';}while(0)
// The out-of-date function
#define	asctime(...)	asctime_s(...)
#define	ctime(...)		ctime_s(...)
#define	strlen(a)	mystr_len(a)

// for stdio
#define INPUT(str) do{char *p;fgets(str,sizeof(str),stdin);p=strchr(str,'\n');if(p)*p='\0';}while(0)
#define REP(a,b) for(int a=0;a<(int)(b);++a)
#define lolong long long
#define INPBUF (250+2)


// ***********************
// 外部変数

// ***********************
int main() {
	char	str[INPBUF];
	double	ABCD,AP,CQ,W;
	double ans;
	INPUT( str );
	sscanf( str," %lf %lf %lf %lf", &ABCD, &AP, &CQ, &W);

	ans = ( ABCD - AP * CQ ) / 2.0;
	printf("%.10lf\n", ans);

	return 0;
}