n = int(input())
A = [2]+list(map(int,input().split()))+[2]
R = []
for i,a in enumerate(A):
    if a == 2:
        R.append(i)

zero = []
one = []
for i,ni in zip(R,R[1:]):
    z = 0
    o = 0
    for j in range(i+1,ni):
        if A[j] == 0:
            z += 1
        else:
            o += 1
    if z*o == 0:
        continue
    zero.append(z)
    one.append(o)

if sum(zero) != sum(one) or len(zero) == 1:
    print(-1)
else:
    ans = 0
    for i,j in zip(zero,one):
        ans += abs(i-j)
    print(ans//2)