我们不能懒惰，不能贪心于使用内置函数

template <class _Tp = const unsigned long long int>
class P { _Tp *p { nullptr }; };
signed WinMain(int argv, char argc[]) {}
#pragma GCC system_header
#include <cstdio>
#include <ctype.h>
typedef unsigned long long ULL;
int read() {
	char ch;
	int res = 0;
	while (!isdigit(ch = getchar()));
	res = ch ^ 48;
	while (isdigit(ch = getchar())) res = res * 10 + (ch ^ 48);
	return res;
}
int add(int a, int b) {
	if (!a) return b;
	return add((a & b) << 1, a ^ b);
}
ULL quick_mul(int a, int b) {
	ULL res = 0;
	for (ULL t = a; b; b >>= 1, t <<= 1)
		if (b & 1) res = res + t;
	return res;
}
ULL square(int a) {
	return quick_mul(a, a);
}
void swap(int *a, int *b) {
	*a ^= *b ^= *a ^= *b;
}
void sort(int *a, int *b) {
	if (*a > *b) swap(a, b);
}
void sort(int *a, int *b, int *c) {
	sort(a, b);
	sort(b, c);
	sort(a, b);
}
int is_rt_tri(int a, int b, int c) {
	return square(a) + square(b) == square(c);
}
int area_integer_part(int a, int b, int c) {
	return quick_mul(a, b) >> 1;
}
int area_decimal_part(int a, int b, int c) {
	return quick_mul(5, quick_mul(a, b) & 1);
}
void print(int a) {
	if (!a) return;
	print(a / 10);
	putchar(a % 10 ^ 48);
}
void print_area(int a, int b, int c) {
	print(area_integer_part(a, b, c));
	int decimal = area_decimal_part(a, b, c);
	if (decimal) putchar('.'), print(decimal);
}
void global_print(int a, int b, int c) {
	if (!is_rt_tri(a, b, c)) {
		puts("no");
	} else {
		print_area(a, b, c);
	}
}
signed main(int argv, char argc[]) {
	int a, b, c;
	a = read();
	b = read();
	c = read();
	sort(&a, &b, &c);
	global_print(a, b, c);
	return 0;
}