import sys

def main():
    N = int(input())
    spaces_list = []
    odd = False
    even = False
    for i in range(N):
        space, tab = map(int, input().split())
        spaces = space + 4 * tab
        if spaces % 2 == 0:
            even = True
        else:
            odd = True

        if odd and even:
            print(-1)
            sys.exit()
        spaces_list.append(spaces)
    #print(spaces_list)

    if len(set(spaces_list)) == 1:
        print(0)

    max_spaces = max(spaces_list)
    needed_spaces = 0
    for spaces in spaces_list:
        needed_spaces += max_spaces - spaces

    print(int(needed_spaces / 2))


if __name__ == '__main__':
    main()