n = int(input())
a = []
b = []
for i in range(n):
    ai,bi=[int(i) for i in input().split()]
    a.append(ai)
    b.append(bi)
    
def max(a, b):
    if a >= b:
        return a
    else:
        return b
    
res = 0
target = a[0] + 4*b[0]
for i in range(n):
    if i == 0:
        continue
    cur = a[i] + 4*b[i]
    
    if (target - cur) % 2 == 0:
        target = max(target, cur)
    else:
        res = -1
        break

if res == -1:
    print(res)
else:
    for i in range(n):
        res += target - (a[i] + 4*b[i])
    print(int(res/2))