n = int(input())
a = list(map(int, input().split()))

L = 0
while a[L] == 0 and L < n:
    L += 1
    
if L == n:
    print(0)

else:
    R = L
    while L < n - 1:
        count = 0
        
        while R < n - 1 and a[R] != a[R + 1]:
            R += 1
            count += 1
        
        if R == n - 1 and count >= 1:
            count -= 1

        a[L] += count
        while L < R - 1:
            L += 1
            count -= 1
            if a[L] == 1:
                a[L] += count
        
        if L == R:
            L += 1
            R = L
        
        elif a[R] == 0:
            while R < n - 1 and a[R] == 0:
                R += 1
            L = R
        
        elif a[R] == 1:
            L = R

a[n - 1] = 0
ans = 0
for i in a:
    ans += i
print(ans)