import sys
input = sys.stdin.readline
n = int(input())
candidate = []
for _ in range(n):
candidate.append(int(input()))
cnt = 0
if max(candidate) == candidate[0] and candidate.count(max(candidate)) == 1:
print(0)
else:
while True:
max_candi = -99829323
max_candi_idx = 0
for i in range(1, n):
if candidate[i] > max_candi:
max_candi = candidate[i]
max_candi_idx = i
if max_candi >= candidate[0]:
candidate[max_candi_idx] -= 1
candidate[0] += 1
cnt += 1
if candidate.count(max(candidate)) == 1 and max(candidate) == candidate[0]:
print(cnt)
break