Language: C++

#include <bits/stdc++.h>
 
using namespace std;
 
typedef long long ll;
 
const int N = 1e6 + 7;
 
unordered_set <int> rw[N];
unordered_set <int> cl[N];
 
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, m, k;
    cin >> n >> m >> k;
    int x1, y1, x2, y2;
    cin >> x1 >> y1 >> x2 >> y2;
    x1--, y1--, x2--, y2--;
    vector <vector <char> > c;
    vector <vector <int> > up, dw, lf, rg, d;
    c.resize(n);
    up.resize(n);
    dw.resize(n);
    lf.resize(n);
    rg.resize(n);
    d.resize(n);
    for (int i = 0; i < n; i++) {
        c[i].resize(m);
        up[i].resize(m);
        dw[i].resize(m);
        lf[i].resize(m);
        rg[i].resize(m);
        d[i].resize(m);
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> c[i][j];
        }
    }
    for (int i = 0; i < n; i++) {
        int lst = -1;
        for (int j = 0; j < m; j++) {
            if (c[i][j] == '@') {
                lst = j;
            }
            lf[i][j] = lst;
        }
    }
    for (int i = 0; i < n; i++) {
        int lst = m;
        for (int j = m - 1; j >= 0; j--) {
            if (c[i][j] == '@') {
                lst = j;
            }
            rg[i][j] = lst;
        }
    }
    for (int j = 0; j < m; j++) {
        int lst = -1;
        for (int i = 0; i < n; i++) {
            if (c[i][j] == '@') {
                lst = i;
            }
            up[i][j] = lst;
        }
    }
    for (int j = 0; j < m; j++) {
        int lst = n;
        for (int i = n - 1; i >= 0; i--) {
            if (c[i][j] == '@') {
                lst = i;
            }
            dw[i][j] = lst;
        }
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            rw[i].insert(j);
            cl[j].insert(i);
        }
    }
    const int inf = 1e9 + 7;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            d[i][j] = inf;
        }
    }
    queue <pair <int, int> > q; 
    d[x1][y1] = 0;
    q.push({x1, y1});
    rw[x1].erase(y1);
    cl[y1].erase(x1);
    int cnt = 0;
    while (!q.empty()) {
        cnt++;
        if (cnt > 1e6) assert(0);
        int x = q.front().first;
        int y = q.front().second;
        q.pop();
        //up
        {
            int lim = max(x - k, up[x][y] + 1);
            int cur = x;
            while (true) {
                if (cl[y].empty()) break;
                auto to = lower_bound(cl[y].begin(), cl[y].end(), cur);
                if (to == cl[y].begin()) {
                    break;
                }
                to--;
                if (*to < lim) {
                    break;
                }
                d[*to][y] = d[x][y] + 1;
                cur = *to;
                q.push({*to, y});
                rw[*to].erase(y);
                cl[y].erase(*to);
            }
        }
        //down
        {
            int lim = min(x + k, dw[x][y] - 1);
            int cur = x;
            while (true) {
                if (cl[y].empty()) break;
                auto to = upper_bound(cl[y].begin(), cl[y].end(), cur);
                if (to == cl[y].end()) {
                    break;
                }
                if (*to > lim) {
                    break;
                }
                d[*to][y] = d[x][y] + 1;
                cur = *to;
                q.push({*to, y});
                rw[*to].erase(y);
                cl[y].erase(*to);
            }
        }
        //left
        {
            int lim = max(y - k, lf[x][y] + 1);
            int cur = y;
            while (true) {
                if (rw[x].empty()) break;
                auto to = lower_bound(rw[x].begin(), rw[x].end(), cur);
                if (to == rw[x].begin()) {
                    break;
                }
                to--;
                if (*to < lim) {
                    break;
                }
                d[x][*to] = d[x][y] + 1;
                cur = *to;
                q.push({x, *to});
                cl[*to].erase(x);
                rw[x].erase(*to);
            }
        }
        //right
        {
            int lim = min(y + k, rg[x][y] - 1);
            int cur = y;
            while (true) {
                if (rw[x].empty()) break;
                auto to = upper_bound(rw[x].begin(), rw[x].end(), cur);
                if (to == rw[x].end()) {
                    break;
                }
                if (*to > lim) {
                    break;
                }
                d[x][*to] = d[x][y] + 1;
                cur = *to;
                q.push({x, *to});
                cl[*to].erase(x);
                rw[x].erase(*to);
            }
        }
    }
    cout << (d[x2][y2] == inf ? -1 : d[x2][y2]) << '\n';
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:111:19: error: no 'operator--(int)' declared for postfix '--' [-fpermissive]
  111 |                 to--;
      |                 ~~^~
a.cc:152:19: error: no 'operator--(int)' declared for postfix '--' [-fpermissive]
  152 |                 to--;
      |                 ~~^~