Codeforces Round #FF (Div. 2), problem: (A) DZY Loves Hash solution

Codeforces Round #FF (Div. 2), problem: (A) DZY Loves Hash: http://codeforces.com/contest/447/problem/A

Codeforces Round #FF (Div. 2), problem: (A) DZY Loves Hash editorial: http://codeforces.com/blog/entry/13036

Codeforces Round #FF (Div. 2), problem: (A) DZY Loves Hash solution: http://ideone.com/EZ6IdH

#include <iostream>
#include <cstdio>
using namespace std;

int main() {
    int p, n, x, a[305], idx=-1;
    for(int i=0; i<305; i++) a[i]=0;
    scanf("%d%d", &p, &n);
    for(int i=0; i<n; i++) {
        scanf("%d", &x); 
        if(a[x%p]==0) a[x%p]++;
        else {
            idx=i+1;
            break;
        }
    }
    printf("%d", idx);
    return 0;
}