import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N = int(readline())
m = map(int,read().split())
W = [x+4*y for x,y in zip(m,m)]

max_w = max(W)
if any((max_w-x)&1 for x in W):
    print(-1)
else:
    answer = sum(max_w-x for x in W) // 2
    print(answer)