N=int(input())
X=[0]*N
Y=[0]*N
cnt=0
for _ in range(N):
    X[_],Y[_]=list(map(int,input().split()))
k=Y[0]-X[0]
if 0<=k and k<=N:
    for i in range(N):
        cnt+=(Y[i]-X[i])
    if cnt==k*N:
        print(k)
    else:
        print(-1)
else:
    print(-1)