#include <cstdio>
#include <algorithm>

using namespace std;

int N, M, sol;
bool gotov = false;
pair< int, int > D[ 100010 ];

void smanji( int poc, int kraj, int k ) {
	for( int i = poc; i <= kraj; ++ i )
		D[ i ].second -= k;
}

int main( void ) {
	scanf( "%d %d", &N, &M );
	for( int i = 0; i < N; ++ i )
		scanf( "%d %d", &D[ i ].first, &D[ i ].second );	
	
	sort( D, D + N );
	
	while( !gotov ) {
/*		for( int i = 0; i < N; ++ i ) printf( "%d ", D[ i ].second );
		printf( "\n" );*/
		
		int min = -1, pocetak; gotov = true;
		for( int i = N - 1; i >= -1; -- i ) {
			if( min != -1 && ( i == -1 || D[ i ].second == 0 ) ) { smanji( i + 1, pocetak, min ); sol += min; break; }
			else if( min == -1 && D[ i ].second > 0 ) { min = D[ i ].second; pocetak = i; gotov = false; }
			else if( min != -1 && D[ i ].second < min ) min = D[ i ].second;
		}	
	}	
	
	printf( "%d\n", sol );
	return 0;	
}