C++ :

#include<iostream>
#include<string>
#include<cstring>
using namespace std;
const int maxn=20+5;
const int maxm=20+5;
int n,m,ans;
string map[maxn];
bool visited[maxn][maxm];
void search(int row,int col)
{
    if(row<0 || row>=n || col<0 || col>=m || map[row][col]=='#' || visited[row][col])
        return;
    visited[row][col]=true;
    ++ans;
    search(row-1,col);
    search(row+1,col);
    search(row,col-1);
    search(row,col+1);
}
int main()
{
    int row,col,i,j;
    cin>>n>>m;
    for(i=0;i<n;i++){
        cin>>map[i];
        for(j=0;j<m;j++){
            if(map[i][j]=='@'){
                row=i;
                col=j;
            }
        }
    }
    //cout<<row<<" "<<col<<endl;
    memset(visited,false,sizeof(visited));
    ans=0;
    //visited[row][col]=1;
    search(row,col);
    cout<<ans-1<<endl;
    return 0;
}