Language: C++

 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#include <iomanip>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << endl
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
int dx8[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy8[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;


//ポイントは先に経路固定→bfsと思いつくこと、でその最短のdist=最低限使う白の個数
//今回みたいなスタートとゴールがあり、最初か、途中かに盤面をいじれる場合
//①探索しながら盤面変化、②先に経路固定してそのコストを考える
int H, W;
string fi[110];
vvi dist[110][110] = {-1}; // BFS 用

int main() {
    // 入力
    cin >> H >> W;
    int wnum = 0;
    for (int i = 0; i < H; ++i) {
        cin >> fi[i];
        for (int j = 0; j < W; ++j) {
            if (fi[i][j] == '.') ++wnum;
        }
    }

    
    dist[0][0] = 1;
    queue<pii> que;
    que.push(pii(0, 0));
    while (!que.empty()) {
        pii cur = que.front();
        que.pop();
        int x = cur.first;
        int y = cur.second;

        for(auto dx:dx4){
            for(auto dy:dy4){
            int nx = x + dx;
            int ny = y + dy;
            if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
            if (fi[nx][ny] == '#') continue;
            if (dist[nx][ny] == -1) {
                dist[nx][ny] = dist[x][y] + 1;
                que.push(pii(nx, ny));
            }
            }
        }
    
    }
    if (dist[H-1][W-1] == -1) cout << -1 << endl;
    else  cout << wnum - dist[H-1][W-1] << endl;
}

Compiler output:
a.cc:65:23: error: conversion from 'int' to non-scalar type 'vvi' {aka 'std::vector<std::vector<int> >'} requested
   65 | vvi dist[110][110] = {-1}; // BFS 用
      |                       ^~
a.cc: In function 'int main()':
a.cc:79:18: error: no match for 'operator=' (operand types are 'vvi' {aka 'std::vector<std::vector<int> >'} and 'int')
   79 |     dist[0][0] = 1;
      |                  ^
In file included from /usr/include/c++/14/vector:72,
                 from a.cc:5:
/usr/include/c++/14/bits/vector.tcc:210:5: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = std::vector<int>; _Alloc = std::allocator<std::vector<int> >]'
  210 |     vector<_Tp, _Alloc>::
      |     ^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:211:42: note:   no known conversion for argument 1 from 'int' to 'const std::vector<std::vector<int> >&'
  211 |     operator=(const vector<_Tp, _Alloc>& __x)
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/14/vector:66:
/usr/include/c++/14/bits/stl_vector.h:766:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = std::vector<int>; _Alloc = std::allocator<std::vector<int> >]'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:766:26: note:   no known conversion for argument 1 from 'int' to 'std::vector<std::vector<int> >&&'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |                 ~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:788:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_Tp>) [with _Tp = std::vector<int>; _Alloc = std::allocator<std::vector<int> >]'
  788 |       operator=(initializer_list<value_type> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:788:46: note:   no known conversion for argument 1 from 'int' to 'std::initializer_list<std::vector<int> >'
  788 |       operator=(initializer_list<value_type> __l)
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
a.cc:94:30: error: no match for 'operator==' (operand types are 'vvi' {aka 'std::vector<std::vector<int> >'} and 'int')
   94 |             if (dist[nx][ny] == -1) {
      |                 ~~~~~~~~~~~~ ^~ ~~
      |                            |    |
      |                            |    int
      |                            vvi {aka std::vector<std::vector<int> >}
In file included from /usr/include/c++/14/iosfwd:42,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/postypes.h:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::fpos<_StateT>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
In file included from /usr/include/c++/14/string:43,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44:
/usr/include/c++/14/bits/allocator.h:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::allocator<_CharT>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::reverse_iterator<_Iterator>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::reverse_iterator<_Iterator>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::move_iterator<_IteratorL>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::move_iterator<_IteratorL>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::pair<_T1, _T2>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:629:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  629 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:629:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'std::vector<std::vector<int> >' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/string_view:637:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  637 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:637:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'std::vector<std::vector<int> >' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/string_view:644:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  644 |     operator==(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:644:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3755:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3755 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3755:5: note:   template argument deduction/substitution failed:
a.cc:94:34: note:   'vvi' {aka 'std::vector<std::vector<int> >'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   94 |             if (dist[nx][ny] == -1) {
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3772:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3772 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& _