#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>
#define X first
#define Y second

using namespace std;

int main() {
    int n, m;   scanf( "%d %d", &n, &m );
    pair<int, int> inp[n];
    for( int i = 0; i < n; i++ ) {
         int p, c;
         scanf( "%d %d", &p, &c );
         inp[i] = make_pair( p, c );
    }
    
    sort( inp, inp+n );
    
    int rusenje = 0;
    
    for( int i = 0; i < n; i++ ) {
         rusenje += inp[i].Y;
         for( int j = i+1; j < n, rusenje; j++ ) {
              if( rusenje >= inp[j].Y ) {
                  rusenje -= inp[j].Y;
                  inp[j].Y = 0;
              }
              else {
                  inp[j].Y -= rusenje;
                  rusenje = 0;
              }
         }         
    }
    
    int outp = 0;
    for( int i = 0; i < n; i++ )
         outp += inp[i].Y;
    
    printf( "%d\n", outp );
    scanf( "%%" );
    return 0;
}