local _i={}
for i=1,1 do
    local a=io.read()
    _i[i]={}
    for s in string.gmatch(a,"%S+")do
        _i[i][#_i[i]+1]=s
    end
end

local a={tonumber(_i[1][1]),tonumber(_i[1][2]),tonumber(_i[1][3])}
table.sort(a)
print(a[1]*a[2]*.5)