N = int(input())
edge = []
for _ in range(N):
    a,b = map(int,input().split())
    edge.append((a,b))
_max = edge[0][0] + edge[0][1] * 4
import sys
for a,b in edge:
    if (_max + a + b * 4) & 1:
        print(-1)
        exit()
    if a + b * 4 > _max:
        _max = a + b * 4
ans = 0
for a,b in edge:
    tmp = _max - a - b * 4
    ans += tmp // 2
print(ans)