C++ :

#include <bits/stdc++.h>
using namespace std;
long long f[300010],sumt[300010],sumc[300010];
int q[300010],n,s;
int main(){
   cin>>n>>s;
    for(int i=1;i<=n;i++)
    {
      int t,c;cin>>t>>c;
      sumt[i]=sumt[i-1]+t;
      sumc[i]=sumc[i-1]+c;
      }
   memset(f,0x3f,sizeof(f));
    int l=1,r=1;
   f[0]=0,   q[l]=0;
    for(int i=1;i<=n;i++)
    {
      while(l < r && (f[q[l+1]]-f[q[l]]) <= (s+sumt[i])*(sumc[q[l+1]] - sumc[q[l]])) l++;
      f[i]=f[q[l]] - (s+sumt[i])*sumc[q[l]] + sumt[i]*sumc[i] + s*sumc[n];
      while(l < r && (f[q[r]] - f[q[r-1]]) * (sumc[i] -sumc[q[r]]) >= (f[i]-f[q[r]]) * (sumc[q[r]] - sumc[q[r-1]])) r--;
      q[++r] =i;
      }
   cout<<f[n]<<endl;
    return 0;
}