#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int min_moves_to_sort_cars(int N, vector<int>& P) {
    // Find the position where sorting starts
    int pos = 0;
    for (int i = 1; i < N; ++i) {
        if (P[i] < P[i - 1]) {
            pos = i;
            break;
        }
    }

    // Check if already sorted
    if (pos == 0) return 0;

    // Check if sorting is possible
    for (int i = pos + 1; i < N; ++i) {
        if (P[i] < P[i - 1]) return -1;
    }
    if (P[N - 1] > P[0]) return -1;

    return N - pos;
}

int main() {
    int N;
    cin >> N;
    vector<int> P(N);
    for (int i = 0; i < N; ++i) {
        cin >> P[i];
    }

    int result = min_moves_to_sort_cars(N, P);
    cout << result << endl;

    return 0;
}