#include<iostream>
#include<cstdio>
using namespace std;
#define re register
#define Re register
#define MAXN 5005
int n,S,opt[MAXN];
struct task{
    int t,c;
}a[MAXN],s[MAXN];
int main() {
    scanf("%d%d",&n,&S);
    for(re int i = 1;i <= n;i++){
        scanf("%d%d",&a[i].t,&a[i].c);
        s[i].t = s[i - 1].t + a[i].t;
        s[i].c = s[i - 1].c + a[i].c;
    }
    for(re int i = 1;i <= n;i++){
        opt[i] = 2e9;
        for(re int j = 0;j < i;j++){
            opt[i] = min(opt[i],opt[j] + s[i].t * (s[i].c - s[j].c) + S * (s[n].c - s[j].c));
        }
    }
    cout << opt[n] << endl;
    return 0;
}