#include<bits/stdc++.h>
const int N=1e6+5;
//const ll mod=998244373;
using namespace std;

int n,t;
int m;
string s;
//int x,y;
int ans;
int head[N],cnt;
int vis[N];

struct edge{
   	int next;
   	int to;
}e[N];

void add(int from,int to)
{
	e[++cnt].next=head[from];
	e[cnt].to=to;
	head[from]=cnt;
}

void dfs(int x,int num)
{
	vis[x]=1;
	for(int i=head[x];i;i=e[i].next)
	  {
	  	int y=e[i].to;
	  	if(vis[y])
	  	  continue;
	  	if(num==3&&s[y]=='o')
	  	  {
	  	  	++ans;
	  	  	continue;
	  	  }
	    if(num==2&&s[y]=='b')
	      	dfs(y,3);
	    if(num==1&&s[y]=='o')
	      dfs(y,2);
	  }
	vis[x]=0;
}

int main()
{
	//ios_base::sync_with_stdio(0);
    //cin.tie(0);
	while(scanf("%d%d",&n,&m)!=EOF)
	  {
	    cin>>s;
	    s=" "+s;
	    cnt=0;
	    for(int i=1;i<=n+5;++i)
	      head[i]=0;
	    ans=0;
	    while(m--)
	      {
	      	int x,y;
	      	scanf("%d%d",&x,&y);
	      	//cin>>x>>y;
	      	add(x,y);
	      	add(y,x);
	      }
	    for(int i=1;i<=n;++i)
	      {
	      	if(s[i]=='b')
	      	  dfs(i,1);
	      }
	    printf("%d\n",ans);
     }
}