text: #python 3.5.2
  
  import sys 
  while True:
  
      try:
          line = input().split()
          list2 = []
          flag= -1
          #print(int(line[1]))
          for i in range(int(line[1])):
              k = input()
              list2.append(int(k))
          #print(list2)
          a = [0]*int(line[0])
          b = int(line[1])
          #print(line[0])
          for i in range(0, b):
              k = list2[i] % len(a)
              if a[k] == 0:
                  a[k] = 1
                  #print(k)
              else:
                  flag = i+1
                  break
          print(flag)
      except:
          break