n,k=[int(x) for x in input().split()]
e=0
l=[]
for i in range(n):
a,b=[int(x) for x in input().split()]
if b>k:
e=a-(b-k)
else:
e=a
l.append(e)
print(max(l))#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main()
{
ll n,k,i=0,marc;
cin>>n>>k;
while(i<n)
{
ll a,b;
cin>>a>>b;
if(i==0)
{
if(b>k) marc=a-(b-k);
else marc=a;
}
else{
ll joya;
if(b>k) joya=a-(b-k);
else joya=a;
if(joya>marc)marc=joya;
}
i++;
}
cout<<marc<<"\n";
}
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