n,d=map(int,input().split())
p=[]
q=[]
for i in range(n):
    a,b=map(int,input().split())
    p.append(a)
    q.append(b)
def f(x):
    res=0
    f=-1
    for _ in range(d):
        up=-10**18
        ind=-1
        for i in range(n):
            if i!=f and res-p[i]>=x:
                up=max(up,q[i]-p[i])
                if up==q[i]-p[i]:ind=i

        if up == -10 ** 18: return False
        res+=up
        f=ind
    return True

ok=-10**18
ng=0
while ng-ok>1:
    mid=(ok+ng)//2
    if f(mid):ok=mid
    else:ng=mid
print(ok)