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):
    dp=[-10**18]*n
    for i in range(n):
        if -p[i]>=x:dp[i]=-p[i]+q[i]
    maxdp1=[-10**18]*(n+1)
    maxdp2=[-10**18]*(n+1)
    for i in range(n):
        maxdp1[i]=max(maxdp1[i-1],dp[i])
    for i in range(n-1,-1,-1):
        maxdp2[i]=max(maxdp2[i+1],dp[i])
    for _ in range(d-1):
        for i in range(n):
            res=max(maxdp1[i-1],maxdp2[i+1])
            if res-p[i]>=x:
                dp[i]=res-p[i]+q[i]
            else:dp[i]=-10**18

        for i in range(n):
            maxdp1[i] = max(maxdp1[i - 1], dp[i])
        for i in range(n - 1, -1, -1):
            maxdp2[i] = max(maxdp2[i + 1], dp[i])
    for i in range(n):
        if dp[i]>-10**18:return True
    return False
ok = -10 ** 18
ng = 0
while ng - ok > 1:
            mid = (ok + ng) // 2
            if f(mid):
                ok = mid
            else:
                ng = mid
print(ok)