from collections import *
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def MF(): return map(float, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LF(): return list(map(float, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]

def main():
    n=II()
    ww=[a+4*b for a,b in [LI() for _ in range(n)]]
    ans=0
    mx=max(ww)
    for w in ww:
        if (mx-w)%2:
            print(-1)
            exit()
        else:
            ans+=(mx-w)//2
    print(ans)

main()