#python
sxy, sx, sy, sx2=0, 0, 0, 0
for _ in range(5):
    x, y=list(map(int, input().split()))
    sx+=x
    sy+=y
    sxy+=x*y
    sx2+=x**2
b=(5*sxy-sx*sy)/(5*sx2-sx**2)
a=(sy-b*sx)/5
print(round(a+b*80, 3))