#include<bits/stdc++.h>
using namespace std;
#define rg register
#define ll long long
#define ull unsigned ll
#define lowbit(x) (x&(-x))
int djq;
const double eps=1e-10;
const short sint=0x3f3f;
const int inf=0x3f3f3f3f;
const ll linf=0x3f3f3f3f3f3f3f3f;
const double alpha=0.73;
const double PI=acos(-1);
inline void file(){
	freopen("entrench.in","r",stdin);
	freopen("entrench.out","w",stdout);
}
inline ll read(){
	rg ll ret=0,f=0;char ch=getchar();
    while(!isdigit(ch)){if(ch==EOF)exit(0);if(ch=='-')f=1;ch=getchar();}
    while(isdigit(ch)){ret=ret*10+ch-48;ch=getchar();}
    return f?-ret:ret;
}

#define ep emplace
#define epb emplace_back
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define it iterator
#define mkp make_pair
#define naive return 0*puts("Yes")
#define angry return 0*puts("No")
#define fls fflush(stdout)
#define rep(i,x,y) for(rg int i=(x);i<=(y);++i)
#define per(i,x,y) for(rg int i=(x);i>=(y);--i)
#define rep0(i,a) for(rg int i=0;i<=a;++i)
#define per0(i,a) for(rg int i=a;~i;--i)
#define szf sizeof
typedef vector<int> vec;
typedef pair<int,int> pii;
struct point{ int x,y; point(int x=0,int y=0):x(x),y(y) {} inline bool operator<(const point& T)const{ return x^T.x?x<T.x:y<T.y; }; };
inline int ksm(int base,int p){int ret=1;while(p){if(p&1)ret=1ll*ret*base%djq;base=1ll*base*base%djq,p>>=1;}return ret;}
inline void pls(int& x,const int k){ x=(x+k>=djq?x+k-djq:x+k); }
inline int add(const int a,const int b){ return a+b>=djq?a+b-djq:a+b; }
inline void sub(int& x,const int k){ x=(x-k<0?x-k+djq:x-k); }
inline int inc(const int a,const int b){ return a<b?a-b+djq:a-b; }
inline void ckmn(int& x,const int k){ x=(k<x?k:x); }
inline void ckmx(int& x,const int k){ x=(k>x?k:x); }
inline void ckmn(ll& x,const ll k){ x=(k<x?k:x); }
inline void ckmx(ll& x,const ll k){ x=(k>x?k:x); }
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());

int n,a[300005],b[300005],ans;
void dif(int x){
	rep(i,0,n-1) b[i]=inc(a[(i+x)%n],a[i]);
}
int chk(int x){
	rep(i,0,n-1) if(a[i]!=a[(i+x)%n]) return 0;
	return 1;
}
signed main(){
//	file();
	n=read(),djq=read();
	rep(i,0,n-1) a[i]=read()%djq;
	ans+=(a[0]!=0);
	rep(i,1,n-1) a[i]=inc(a[i],a[0]);
	a[0]=0;
	int nw=1;
	while(n%(nw*djq)==0) nw*=djq;
	if(!chk(nw)) return 0*puts("-1");
	while(nw!=1){
		nw/=djq;
		while(!chk(1)){
			dif(nw);
			if(chk(1)&&nw!=1) break;
			rep(i,0,n-1) a[i]=b[i];
			ans+=nw;
		}
	} 
	printf("%d\n",ans);
	return 0; 
}
/*

*/