package main
import (
"bufio"
"fmt"
"math"
"os"
)
var writer *bufio.Writer
var reader *bufio.Reader
func Print(a ...interface{}) {
fmt.Fprintln(writer, a...)
}
func Solve() {
var w, h, r int
fmt.Fscan(reader, &w, &h, &r)
area := math.Pi * float64(r) * float64(r) / 4.0
Print(float64(w)*float64(h) - area)
}
func main() {
writer = bufio.NewWriter(os.Stdout)
reader = bufio.NewReader(os.Stdin)
defer writer.Flush()
Solve()
}