#include "bits/stdc++.h"
using namespace std;
#ifdef DEBUG
#define dbg(x) cout << (#x) << " = " << (x) << "\n";
#else
#define dbg(x)
#endif

typedef long long ll;
typedef pair<int, int> pi;
const int N = 500005;
const int inf = 1e9;
int n, m, k, qq, d;
int a[N], t[N * 4];
void build(int v = 1, int tl = 0, int tr = n - 1) {
    if (tl == tr) {
        t[v] = a[tl];
        return;
    }
    
    int mid = (tl + tr) / 2;
    build(v * 2, tl, mid);
    build(v * 2 + 1, mid + 1, tr);
    t[v] = min(t[v * 2], t[v * 2 + 1]);
}

void update(int p, int v = 1, int tl = 0, int tr = n - 1) {
    if (tl == tr) {
        t[v] = inf;
        return;    
    }
    
    int mid = (tl + tr) / 2;
    if (p <= mid) update(p, v * 2, tl, mid);
    else update(p, v * 2 + 1, mid + 1, tr);
    t[v] = min(t[v * 2], t[v * 2 + 1]);
}

int findfirst(int p, int x, int v = 1, int tl = 0, int tr = n - 1) {
    if (tr <= p || t[v] > x) return -1;
    if (tl == tr) return tl;
    int mid = (tl + tr) / 2;
    int L = findfirst(p, x, v * 2, tl, mid);
    if (L != -1) return L;
    return findfirst(p, x, v * 2 + 1, mid + 1, tr);
}

void solve() {
    cin >> d >> n;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    
    build();
    int out = 0, p = -1, cur = 0;
    while (cur < n) {
        int px = findfirst(p, cur);
        if (px == -1 && p == -1) {
            cout << "-1\n";
            return;
        }
        
        if (px == -1) out++, p = -1;
        else {
            p = px, cur++;
            update(p);
        }
    }
    
    cout << out + 1 << "\n";
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    solve();
}