def ascan; gets.split.map(&:to_i); end

a,B,c = ascan

A = a*60
C = c*60*60

if A <= B
    p -1
else
    # Ax > Bx+C
    # C < (A-B)x
    # C/(A-B) < x
    z = (C+(A-B-1))/(A-B)
    p z <= 0 ? -1 : (C+(A-B-1))/(A-B)
end