buckets, numbersCount = list(map(int, input().split()))
ans = -1
numbers = []
hashes = []
for _ in range(numbersCount):
numbers.append(int(input()))
counter = 0
for number in numbers:
mod = number % buckets
if not mod in hashes:
hashes.append(mod)
else:
ans = counter + 1
break
counter += 1
print(ans)
#include <bits/stdc++.h>
using namespace std;
int main() {
// your code goes here
long long int p,n;
cin>>p>>n;
long long int arr[n];
for(int i=0;i<n;i++){
cin>>arr[i];
}
long long int ans=-1;
// set<int>s;
vector<long long int>v;
for(int i=0;i<n;i++){
long long int mes=arr[i]%p;
vector<long long int>::iterator it;
it=find(v.begin(),v.end(),mes);
if(it==v.end()){
v.push_back(mes);
}
else{
ans=i+1;
break;
}
}
cout<<ans<<endl;
return 0;
}
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