import sys
INF = float('inf')
#10**20,2**63,float('inf')
MOD = 10**9 + 7
MOD2 = 998244353
#from collections import defaultdict
def solve():
    def II(): return int(sys.stdin.readline())
    def LI(): return list(map(int, sys.stdin.readline().split()))
    def LC(): return list(input())
    def IC(): return [int(c) for c in input()]
    def MI(): return map(int, sys.stdin.readline().split())
    #N=3000,O(N^2)程度は可能。
    N=II()
    P = LI()
    Ans = 0
    for i in range(N):
        n = i+1
        Ans += P[i] - n
        Dis = P[i] - n
        for j in range(i+1,N):
            if(P[j]-n <= Dis):
                P[j] += 1
        P[i] = n
        #print(P)
    print(Ans)
    return
solve()
#sys.setrecursionlimit(10 ** 6)#再帰関数ではコメントにしないこと！！