n = int(input())
alst = list(map(int, input().split()))
l = [alst[0]]
for a in alst[1:-2]:
    l.append(min(l[-1], a))
r = [alst[-1]]
for a in alst[2:-1][::-1]:
    r.append(min(r[-1], a))
r = r[::-1]
inf = 10 ** 10
ans = inf
def kado(a, b, c):
    if b == min(a, b, c) or b == max(a, b, c):
        return True
    else:
        return False

for i, a in enumerate(alst[1:-1]):
    rr = r[i]
    ll = l[i]
    if kado(rr, a, ll):
        ans = min(ans, rr + a + ll)
if ans != inf:
    print(ans)
else:
    print(-1)