N=int (input())
A=[[int (i) for i in input ().split()]for i in range (N)]
nd=0
A[0][1]=A[0][1]*4
A[0][0]+=A[0][1]
M=A[0][0]
i=1
while nd==0 and i<N:
    A[i][1]=A[i][1]*4
    A[i][0]+=A[i][1]
    d=A[i][0]-A[i-1][0]
    if d%2!=0:
        nd=1
        print (-1)
    if M<A[i][0]:
        M=A[i][0]
    i+=1
if nd==0:
    di=0
    for i in range(N):
        di+=M-A[i][0]
    di//=2
    print (di)