C++ :

#include<bits/stdc++.h>
#define R register
using namespace std;
int w,h,s1,s2;
char a[120][120];
int sum;
void dfs(int x,int y){
    if(x<1||y<1||x>h||y>w||a[x][y]=='#')
		return;
	sum++;	
	a[x][y]='#';
	dfs(x+1,y);
	dfs(x-1,y);
	dfs(x,y-1);
	dfs(x,y+1);
}
template <typename T> void read(T &t){
	t=0;
	char ch=getchar();
	int f=1;
	while(ch<'0'||ch>'9'){
		if(ch=='-')f=-1;
		ch=getchar();
	}
	do{
		(t*=10)+=ch-'0';
		ch=getchar();
	}while(ch>='0'&&ch<='9');
	t*=f;
}
int main(){
		sum=0;
		read(w);
		read(h);
		for(R int i=1;i<=h;i++){
			for(R int j=1;j<=w;j++){
				cin>>a[i][j];
				if(a[i][j]=='@'){
					s1=i;
					s2=j;
				}
			}
		}
        dfs(s1,s2);
        printf("%d\n",sum);
	return 0;
}

Python :

def dfs(x,y):
    b[x][y]=1
    a[x][y]="#"
    for i in range(4):
        tx=x+tr[i][0]
        ty=y+tr[i][1]
        if(tx>=0 and tx<h and ty>=0 and ty<w):
            if(b[tx][ty]==0 and a[tx][ty]=="."):
                dfs(tx,ty)
    return
sr=input().split()
w=int(sr[0])
h=int(sr[1])
a=[[0 for i in range(w)] for j in range(h)]
b=[[0 for i in range(w)] for j in range(h)]
tr=[[-1,0],[1,0],[0,-1],[0,1]]
x=0
y=0
for i in range(h):
    sr=input()
    for j in range(w):
        a[i][j]=sr[j:j+1]
        if(a[i][j]=="@"):
            x=i
            y=j
dfs(x,y)
k=0
for i in range(h):
    for j in range(w):
        if(b[i][j]==1):
            k=k+1
print(k)