N = int(raw_input())

max = None
ans = 0

for i in range(N):
    spc, tab = map(int, raw_input().split())
    length = spc + 4 * tab
    if max is None:
        max = length
    elif (max - length)%2 == 1:
        ans = -1
        break
    else:
        ans += abs(max - length)/2
    
print ans