#include <map>
#include <ctime>
#include <cmath>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#define spause() system("pause")

using namespace std;
typedef long long llong;
typedef unsigned long long ullong;
typedef pair<int, int> prdd;

const int dinf = 0x7fffffff;
const llong llinf = 0x7fffffffffffffff;
const int way[4][2] = { { 1, 0 }, { -1, 0 }, { 0, 1 }, { 0, -1 } };

struct cao {
    bool vd, fnd[1024];
    int pic;
};

struct bfnt {
    int f, s, ti;
};

int n, m, res = -1, ans = 0, picno = 0;
vector<bfnt> bfsn, tmp;
cao ecao[32][32];

void bfsnp(bfnt p) {
    for (int i = 0; i < 4; i++) {
        int nl = p.f + way[i][0], nr = p.s + way[i][1];
        if (nl < 0 || nl >= n || nr < 0 || nr >= m || !ecao[nl][nr].vd)
            continue;
        int nti = p.ti;
        if (ecao[nl][nr].pic >= 0)
            nti |= 1 << ecao[nl][nr].pic;
        if (!ecao[nl][nr].fnd[nti]) {
            ecao[nl][nr].fnd[nti] = true;
            tmp.push_back(bfnt{ nl, nr, nti });
            if (nti == (1 << picno) - 1) {
                res = ans;
                bfsn.clear();
                tmp.clear();
            }
        }
    }
}

int main() {
    cin.sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++) {
            char tmp;
            cin >> tmp;
            if (tmp == '#')
                ecao[i][j].vd = false;
            else
                ecao[i][j].vd = true;
            if (tmp == 'p') {
                bfsn.push_back(bfnt{ i, j, 0 });
                ecao[i][j].fnd[0] = true;
            }
            if (tmp == '@')
                ecao[i][j].pic = picno++;
            else
                ecao[i][j].pic = -1;
        }

    while (!bfsn.empty()) {
        ans++;
        for (int i = 0; i < bfsn.size(); i++) bfsnp(bfsn.at(i));
        swap(bfsn, tmp);
        tmp.clear();
    }

    printf("%d", res);

    spause();
    return 0;
}