#include<bits/stdc++.h>
#define R register
#define fp(i,a,b) for(R int i=a,I=b+1;i<I;++i)
#define fd(i,a,b) for(R int i=a,I=b-1;i>I;--i)
#define go(u) for(int i=head[u],v=e[i].v;i;i=e[i].nx,v=e[i].v)
using namespace std;
char buf[1<<21],*p1=buf,*p2=buf;
inline char getc(){return p1==p2&&(p2=(p1=buf)
+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++;}
int read(){
R int res,f=1;R char ch;
while((ch=getc())>'9'||ch<'0')(ch=='-')&&(f=-1);
for(res=ch-'0';(ch=getc())>='0'&&ch<='9';res=res*10+ch-'0');
return res*f;
}
const int N=2005,P=998244353;
inline int add(R int x,R int y){return x+y>=P?x+y-P:x+y;}
inline int dec(R int x,R int y){return x-y<0?x-y+P:x-y;}
inline int mul(R int x,R int y){return 1ll*x*y-1ll*x*y/P*P;}
int ksm(R int x,R int y){
R int res=1;
for(;y;y>>=1,x=mul(x,x))if(y&1)res=mul(res,x);
return res;
}
int x[N],y[N],n,ans,k,res;
int main(){
// freopen("testdata.in","r",stdin);
n=read(),k=read();
fp(i,1,n)x[i]=read(),y[i]=read();
fp(i,1,n){
res=1;fp(j,1,n)if(i!=j)res=mul(res,dec(x[i],x[j]));
res=ksm(res,P-2);
fp(j,1,n)if(i!=j)res=mul(res,dec(k,x[j]));
res=mul(res,y[i]),ans=add(ans,res);
}printf("%d\n",ans);return 0;
}