#include <stdio.h>
#define N 1000000
#define INF 0x3f3f3f3f3f3f3f3fLL
long long min(long long a, long long b) { return a < b ? a : b; }
unsigned int X = 12345;
int rand_() {
return (X *= 3) >> 1;
}
int xx[N], yy[N], ii[N];
void sort(int *ii, int l, int r) {
while (l < r) {
int i = l, j = l, k = r, i_ = ii[l + rand_() % (r - l)], tmp;
while (j < k)
if (yy[ii[j]] == yy[i_])
j++;
else if (yy[ii[j]] < yy[i_]) {
tmp = ii[i], ii[i] = ii[j], ii[j] = tmp;
i++, j++;
} else {
k--;
tmp = ii[j], ii[j] = ii[k], ii[k] = tmp;
}
sort(ii, l, i);
l = k;
}
}
int pq[N], iq[1 + N], cnt;
int lt(int i, int j) { return xx[i] > xx[j]; }
int p2(int p) {
return (p *= 2) > cnt ? 0 : (p < cnt && lt(iq[p + 1], iq[p]) ? p + 1 : p);
}
void pq_up(int i) {
int p, q, j;
for (p = pq[i]; (q = p / 2) && lt(i, j = iq[q]); p = q)
iq[pq[j] = p] = j;
iq[pq[i] = p] = i;
}
void pq_dn(int i) {
int p, q, j;
for (p = pq[i]; (q = p2(p)) && lt(j = iq[q], i); p = q)
iq[pq[j] = p] = j;
iq[pq[i] = p] = i;
}
void pq_add(int i) {
pq[i] = ++cnt, pq_up(i);
}
int pq_remove_first() {
int i = iq[1], j = iq[cnt--];
if (j != i)
pq[j] = 1, pq_dn(j);
pq[i] = 0;
return i;
}
int main() {
int n, k, i;
long long x, ans;
scanf("%d%d", &n, &k);
for (i = 0; i < n; i++)
scanf("%d%d", &xx[i], &yy[i]);
for (i = 0; i < n; i++)
ii[i] = i;
sort(ii, 0, n);
x = 0, ans = INF;
for (i = 0; i < n; i++) {
int i_ = ii[i];
pq_add(i_), x += xx[i_];
if (cnt > k)
x -= xx[pq_remove_first()];
if (cnt == k)
ans = min(ans, x * yy[i_]);
}
printf("%lld\n", ans);
return 0;
}
