#include<algorithm>
#include<bitset>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<deque>
#include<fstream>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<vector>

using namespace std;

#define dbg(x) (cout<<#x<<" = "<<(x)<<'\n')
#ifdef HOME
const string inputFile = "input.txt";
const string outputFile = "output.txt";
#else
const string problemName = "secv";
const string inputFile = problemName + ".in";
const string outputFile = problemName + ".out";
#endif

typedef long long int lld;
typedef pair<int, int> PII;
typedef pair<int, lld> PIL;
typedef pair<lld, int> PLI;
typedef pair<lld, lld> PLL;

const int INF = (1LL << 31) - 1;
const lld LINF = (1LL << 62) - 1;
const int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
const int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};
const int NMAX = 5000 + 5;

int N, P, sol = INF;
int A[NMAX];
int DP[NMAX];
int F[NMAX];

void normalizare() {
    int i;
    int B[NMAX];
    map<int, int> M;

    memcpy(B, A, sizeof(B));
    sort(B + 1, B + N + 1);

    P = unique(B + 1, B + N + 1) - B - 1;

    for(i = 1; i <= P; i++)
        M[B[i]] = i;

    for(i = 1; i <= N; i++)
        A[i] = M[A[i]];
}

int main() {
    int i, j;

#ifndef ONLINE_JUDGE
    freopen(inputFile.c_str(), "r", stdin);
    freopen(outputFile.c_str(), "w", stdout);
#endif

    scanf("%d", &N);

    for(i = 1; i <= N; i++)
        scanf("%d", &A[i]);

    normalizare();

    for(i = 1; i <= N; i++) {
        DP[i] = 1;

        if(A[i] == 1) {
            F[i] = i;
            continue;
        }

        for(j = 1; j <= i - 1; j++)
            if(A[j] + 1 == A[i]) {
                DP[i] = DP[j] + 1;
                F[i] = max(F[i], F[j]);
            }

        if(DP[i] == P)
            sol = min(sol, i - F[i] + 1);
    }

    printf("%d\n", sol == INF ? -1 : sol);

    return 0;
}