#include<cstdio>
#include<string.h>
using namespace std;
int main()
{
    freopen("energii.in","r",stdin);
    freopen("energii.out","w",stdout);
    int n,w,e,c,i,j,max=0;
    scanf("%d\n%d",&n,&w);
    int v[10010];
    memset(v,0,sizeof(v));
    v[1]=1;
    for(i=1;i<=n;i++)
    {
        scanf("%d%d",&e,&c);
        v[e]+=c;
        if(v[e]>max)
            max=v[e];
    }
    printf("%d",max);
    return 0;
}