POJ 2393

#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
    //freopen("acm.acm","r",stdin);
    long long n;
    long long s;
    long long pre_per;
    long long pre_cap;
    long long cur_per;
    long long cur_cap;
    long long sum;
    cin>>n;
    cin>>s;
    cin>>pre_per;
    cin>>pre_cap;
    sum = 0;
    sum += pre_per*pre_cap;
    while(cin>>cur_per>>cur_cap)
    {
        if(pre_per*cur_cap + s*cur_cap < cur_per*cur_cap)
        {
            sum += pre_per*cur_cap + s*cur_cap;
        }
        else
        {
            sum += cur_per*cur_cap;
        }
        pre_per = cur_per;
        pre_cap = cur_cap;
    }
    cout<<sum<<endl;
}