A, B, C = gets.split.map(&:to_i)

if 60*A - B == 0 then
    puts(-1)
end

cross = C*3600 / (60*A - B)
rem = C*3600 % (60*A - B)

if cross >= 0 then
    if rem == 0 then
        p cross
    else
        p cross.floor + 1
    end
else
    p(-1)
end