#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <type_traits>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std;
using namespace placeholders;

using LL = long long;
using ULL = unsigned long long;
using VI = vector< int >;
using VVI = vector< vector< int > >;
using VS = vector< string >;
using ISS = istringstream;
using OSS = ostringstream;
using PII = pair< int, int >;
using VPII = vector< pair< int, int > >;
template < typename T = int > using VT = vector< T >;
template < typename T = int > using VVT = vector< vector< T > >;
template < typename T = int > using LIM = numeric_limits< T >;

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }
template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; }
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); }

#define NUMBERED( name, number ) NUMBERED2( name, number )
#define NUMBERED2( name, number ) name ## _ ## number
#define REP1( n ) REP2( NUMBERED( REP_COUNTER, __LINE__ ), n )
#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2, REP1 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &&e : c )
#define ALL( c ) begin( c ), end( c )
#define AALL( a ) ( remove_all_extents< decltype( a ) >::type * )a, ( remove_all_extents< decltype( a ) >::type * )a + sizeof( a ) / sizeof( remove_all_extents< decltype( a ) >::type )
#define DRANGE( c, p ) begin( c ), begin( c ) + ( p ), end( c )

#define SZ( v ) ( (int)( v ).size() )
#define EXIST( c, e ) ( ( c ).find( e ) != ( c ).end() )

template < typename T > inline bool chmin( T &a, const T &b ){ if ( b < a ) { a = b; return true; } return false; }
template < typename T > inline bool chmax( T &a, const T &b ){ if ( a < b ) { a = b; return true; } return false; }

#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

string get_str()
{
	char buffer[ 1 << 17 ];
	gets( buffer );
	return move( string( buffer ) );
}

int N;
string S;
VVI G;

int dp[ 1 << 17 ][2];

void dfs1( const int u = 0, const int p = -1 )
{
	++dp[u][ S[u] == 'w' ];

	FOR( v, G[u] )
	{
		if ( v == p )
		{
			continue;
		}
		dfs1( v, u );
		dp[u][0] += dp[v][0];
		dp[u][1] += dp[v][1];
	}

	return;
}

LL dfs2( const int u = 0, const int p = -1, const int pc = 0, const int pw = 0 )
{
	VI cs, ws;
	FOR( v, G[u] )
	{
		if ( v == p )
		{
			continue;
		}
		cs.PB( dp[v][0] );
		ws.PB( dp[v][1] );
	}
	cs.PB( pc );
	ws.PB( pw );

	const int tc = accumulate( ALL( cs ), 0 );
	const int tw = accumulate( ALL( ws ), 0 );

	LL res = 0;
	
	if ( S[u] == 'w' )
	{
		res += LL( tc ) * tw;
		REP( i, SZ( cs ) )
		{
			res -= LL( cs[i] ) * ws[i];
		}
	}

	FOR( v, G[u] )
	{
		if ( v == p )
		{
			continue;
		}

		res += dfs2( v, u, tc - dp[v][0] + ( S[u] == 'c' ) , tw - dp[v][1] + ( S[u] == 'w' ) );
	}

	return res;
}

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );
	cout << setprecision( 12 ) << fixed;

	scanf( "%d ", &N );
	S = get_str();
	G.resize( N );

	REP( N - 1 )
	{
		int a, b;
		scanf( " %d%d", &a, &b );
		--a, --b;
		G[a].PB( b );
		G[b].PB( a );
	}

	dfs1();
	printf( "%lld\n", dfs2() );
	fflush( stdout );

	return 0;
}