// #include <bits/stdc++.h>
// using namespace std;
// using ll = long long;
// #define int ll
// #define rep(i, n) for (int i = 0; i < (int)(n); ++i)

#include <bits/stdc++.h>
using ll = long long;
#define int ll
#define FOR(i, a, b) for (int i = (a); i < int(b); ++i)
#define RFOR(i, a, b) for (int i = (b)-1; i >= int(a); --i)
#define rep(i, n) FOR(i, 0, n)
#define rep1(i, n) FOR(i, 1, int(n) + 1)
#define rrep(i, n) RFOR(i, 0, n)
#define rrep1(i, n) RFOR(i, 1, int(n) + 1)
#define all(c) begin(c), end(c)
// const int MOD = 1000000007;

template <typename T>
void __print__(std::ostream &os, const char *, const char *tail, const T &fst) {
    os << fst << tail;
}
template <typename Fst, typename... Rst>
void __print__(std::ostream &os, const char *del, const char *tail, const Fst &fst,
               const Rst &... rst) {
    os << fst << del;
    __print__(os, del, tail, rst...);
}

#ifdef LOCAL
#define dump(...)                                         \
    do {                                                  \
        std::ostringstream os;                            \
        os << __LINE__ << ":\t" << #__VA_ARGS__ << " = "; \
        __print__(os, ", ", "\n", __VA_ARGS__);           \
        std::cerr << os.str();                            \
    } while (0)
#else
#define dump(...)
#endif

template <typename Fst, typename... Rst>
void println(const Fst &fst, const Rst &... rst) {
    __print__(std::cout, "\n", "\n", fst, rst...);
}
template <typename Fst, typename... Rst>
void print(const Fst &fst, const Rst &... rst) {
    __print__(std::cout, " ", "\n", fst, rst...);
}

template <typename iter>
void println_(iter bgn, iter end) {
    while (bgn != end) println(*bgn++);
}

template <typename iter>
void print_(iter bgn, iter end) {
    while (bgn != end) {
        std::cout << *bgn++;
        std::cout << (bgn == end ? "\n" : " ");
    }
}

int _ = (std::cout.precision(10), std::cout.setf(std::ios::fixed), std::cin.tie(0),
         std::ios::sync_with_stdio(0), 0);

template <typename T>
std::vector<T> ndarray(int n, T v) {
    return std::vector<T>(n, v);
}
template <typename... Args>
auto ndarray(int n, Args... args) {
    auto val = ndarray(args...);
    return std::vector<decltype(val)>(n, move(val));
}

template <typename T>
bool umax(T &a, const T &b) {
    return a < b ? a = b, true : false;
}

template <typename T>
bool umin(T &a, const T &b) {
    return a > b ? a = b, true : false;
}

using namespace std;

int n, d;
int t[100010], f[100010];

signed main() {
    while (cin >> n >> d && n) {
        // cout << n << ' ' << d << endl;
        int tcur = 0, pos = 0;
        int man = 0;
        int ans = 0;
        bool ok = true;
        rep(i, n) {
            cin >> t[i] >> f[i];
            --f[i];
        }
        rep(i, n) {
            int trev = pos + f[i];
            int timm = abs(pos - f[i]);
            if (tcur + trev <= t[i] && d) {
                ans += man * pos;
                man = 1;
            } else if (tcur + timm <= t[i] && man + 1 <= d) {
                ans += man * (t[i] - tcur);
                ++man;
            } else {
                ok = false;
                break;
            }
            pos = f[i];
            tcur = t[i];
            dump(ans);
        }
        dump(pos, man);
        ans += pos * man;

        cout << (ok ? ans : -1) << endl;
    }
}

Measured memory_bytes: 4700