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 != 0:
        ans = -1
        break
    elif max > length:
        ans += abs(max - length)/2
    elif max < length:
        ans += abs(max - length)/2 * i
        max = length
    else: continue

print ans