# a is a list, e.g. [2,5,1,2,3,4,7,7,6]
def f(a):
    water = 0
    i = 0
    while i < len(a)-1:
        try:
            tootall = True
            for k in range(i+1,len(a)):
                if a[k] >= a[i]:
                    tootall = False
            if tootall == False:
                try:
                    if a[i] > a[i+1]:
                        x = 0
                        j = 0
                        while j < a[i]:
                            x += 1
                            j = a[i+x]
                        dwater = (x-1)*a[i]
                        for k in range(i+1,i+x):
                            dwater -= a[k]
                        water += dwater
                        i += (x-1)
                except:
                    life = 42
            elif tootall == True:
                tallest = 0
                for b in range(i+1,len(a)):
                    if a[b] > tallest:
                        tallest = a[b]
                        length = b-i-1
                dwater = length*tallest
                for c in range(i+1, length+i+1):
                    dwater -= a[c]
                water += dwater
                i += length
                        
        except:
            life = 42
        i += 1
    return (water)