W = int(input())
N = int(input())
blo = sorted(list(map(int,input().split())))
dig=False
s=0
for i in range(N):
    s+=int(i)
    if i+1<N and s<=W<s+int(i+1):
        print(i)
        break
    elif i+1==N:
        print(i+1)
        break