#include <stdio.h>
#include <algorithm>
#include <map>
#include <vector>

#define MAXN 5000

FILE *fin, *fout;

int v[MAXN + 1];
int v2[MAXN + 1];

int d[MAXN + 1];

bool vis[MAXN + 1];

std::map <int, int> mp;
std::vector <int> neighbors[MAXN + 1];
int k = 0;

void dfs(int nod);

int main() {
    fin = fopen("secv.in", "r");
    fout = fopen("secv.out", "w");

    int N;

    fscanf(fin, "%d", &N);

    for(int i = 0; i < N; i++) {
        fscanf(fin, "%d", &v[i]);
        v2[i] = v[i];
    }

    std::sort(v2, v2 + N);

    for(int i = 0; i < N; i++) {
        if(mp.find(v2[i]) == mp.end()) {
            mp[v2[i]] = ++k;
        }
    }

    for(int i = 0; i < N; i++) {
        v[i] = mp[v[i]];
    }

    for(int i = 0; i < N; i++) {
        for(int j = i + 1; j < N; j++) {
            if(v[j] == (v[i] + 1)) {
                neighbors[i].push_back(j);
            }
        }
    }

    for(int i = 0; i < N; i++) {
        if(!vis[i]) {
            vis[i] = 1;
            dfs(i);
        }
    }

    int ans = MAXN + 1;
    for(int i = 0; i < N; i++) {
        if(v[i] == 1) {
            ans = std::min(ans, d[i]);
        }
    }

    if(ans == (MAXN + 1)) {
        ans = -1;
    }

    fprintf(fout, "%d", ans);
}

inline void dfs(int nod) {
    d[nod] = MAXN + 1;

    if(v[nod] == k) {
        d[nod] = 1;
    }

    for(int i = 0; i < neighbors[nod].size(); i++) {
        int vec = neighbors[nod][i];

        if(!vis[vec]) {
            vis[vec] = 1;
            dfs(vec);
        }

        d[nod] = std::min(d[nod], vec - nod + d[vec]);
    }
}