#include <bits/stdc++.h>
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
namespace IO {
const int L = 1 << 15;
char ibuf[L | 1], *iS = ibuf, *iT = ibuf, obuf[L | 1], *oS = obuf,
                  *oT = obuf + L, c, st[66];
int tp = 0, f;
inline char gc() {
  if (iS == iT) iT = (iS = ibuf) + fread(ibuf, sizeof(char), L, stdin);
  return (*iS++);
}
inline void flush() {
  fwrite(obuf, sizeof(char), oS - obuf, stdout);
  oS = obuf;
}
inline void pc(char c) {
  *oS++ = c;
  if (oS == oT) flush();
}
inline long long read() {
  long long x = 0;
  f = 1, c = gc();
  for (; !isdigit(c); c = gc())
    if (c == '-') f = -1;
  for (; isdigit(c); c = gc()) x = (x << 1) + (x << 3) + (c & 15);
  return x * f;
}
inline void write(long long x) {
  if (!x) pc('0');
  if (x < 0) pc('-'), x = -x;
  while (x) st[++tp] = x % 10 + '0', x /= 10;
  while (tp) pc(st[tp--]);
}
inline void writeln(long long x) {
  write(x);
  pc('\n');
}
inline void wri(long long x) { write(x), pc(' '); }
struct IOflusher {
  ~IOflusher() { flush(); }
} _ioflusher_;
}  // namespace IO
using IO::gc;
using IO::pc;
using IO::read;
using IO::wri;
using IO::write;
using IO::writeln;
using namespace std;
vector<int> vec[105];
int n, T, x, y;
int main() {
  n = read(), T = read();
  for (register int i = (1); i <= int(n); ++i) {
    int x = read();
    vec[x].push_back(read());
  }
  for (register int i = (1); i < int(T); ++i) {
    sort(vec[i].begin(), vec[i].end());
    for (register int j = vec[i].size() - 1; j >= 0; j -= 2)
      if (j - 1 >= 0)
        vec[i + 1].push_back(vec[i][j] + vec[i][j - 1]);
      else
        vec[i + 1].push_back(vec[i][j]);
  }
  sort(vec[T].begin(), vec[T].end());
  writeln(vec[T][vec[T].size() - 1]);
}
