n=int(input())
x=[]
for i in range(n):
    a1,b1=map(int,input().split())
    x.append([a1,b1])
y=[]
for j in range(n):
    y1=x[j][0]+x[j][1]*4
    y.append(y1)
y_max=max(y)
y_sum=0
for k in range(n):
    y_dif=y_max-y[k]
    y_sum=y_sum+y_dif
y_sum2=int(y_sum/2)
if y_sum%2==0:
    print(y_sum2)
else:
    print(-1)