N = int(input()) A = list(map(int, input().split())) for i in range(N - 1): if A[i] < A[i + 1]: break else: print(-1) exit() for i in range(N - 1): if A[i] > A[i + 1]: break else: print(-1) exit() LMIN = [A[0]] * N RMIN = [A[-1]] * N for i in range(1, N): LMIN[i] = min(LMIN[i - 1], A[i]) for i in reversed(range(N - 1)): RMIN[i] = min(RMIN[i + 1], A[i]) ans = 10 ** 10 for i in range(1, N - 1): if LMIN[i - 1] < A[i] > RMIN[i + 1] or LMIN[i - 1] > A[i] < RMIN[i + 1]: ans = min(ans, LMIN[i - 1] + A[i] + RMIN[i + 1]) print(ans)