Untitled

def solution(S):
    n = len(S)
    tanks = 0
    i = 0
    
    while i<n:
        if S[i] == 'H':
            if i < n-1 and S[i+1] == '-':
                tanks += 1
                i += 3
            elif i > 0 and S[i-1] == '-':
                tanks += 1
                i += 1
            else:
                return -1
        else:
            i += 1
            
    return tanks