#include <stdio.h>
#include <stdlib.h>

using namespace std;

int c[100000];
int t[100000];

int main()
{
    FILE *fin, *fout;
    long long n,m,st,dr,mij,i,j,x;
    fin=fopen("garaj.in","r");
    fout=fopen("garaj.out","w");
    fscanf(fin,"%lld%lld",&n,&m);
    for(x=0;x<n;x++){
        fscanf(fin,"%lld%lld",&i,&j);
        c[x]=i;
        t[x]=j;
    }
    st=1;
    dr=1000000000;
    while(st<dr){
        mij=(st+dr)/2;
        x=0;
        for(i=0;i<n;i++){
            x=x+1LL*c[i]*(mij/(t[i]*2));
        }
        if(x>=m)
            dr=mij;
        else
            st=mij+1;
    }
    fprintf(fout,"%lld",st);
    return 0;
}