def main():
    import sys
    input = sys.stdin.read
    data = input().split()
    
    N = int(data[0])
    A = list(map(int, data[1:N+1]))
    
    operations = 0
    
    i = 0
    while i < len(A) - 2:
        if A[i] < A[i+1] and A[i+1] > A[i+2]:
            min_val = min(A[i], A[i+2])
            A[i+1] = min_val - 1
            operations += 1
            i = max(i - 2, 0)  # Check previous positions
        else:
            i += 1
    
    print(operations)

if __name__ == "__main__":
    main()