N = int(input())

A = []

for i in range(N):
    
    a, b = map(int, input().split())
    
    A += [a + 4 * b]
    
X = max(A)

ans = 0

for a in A:
    
    if a % 2 != X % 2:
        
        print(-1)
        exit()
        
    ans += (X - a) // 2
    
print(ans)