#include <stdio.h>
#include <stdlib.h>

struct Help {
    int v, k;
};

struct Queue {
    struct Help *heap;
    int cap, count;
};

struct Queue swap(struct Queue base, int a, int b)
{
    int v = base.heap[a].v, k = base.heap[a].k;
    base.heap[a] = base.heap[b];
    base.heap[b].v = v;
    base.heap[b].k = k;
    return base;
}

struct Queue heapify(int i, int n, struct Queue base)
{
    for (;;) {
        int l = 2 * i + 1;
        int r = l + 1;
        int j = i;
        if (l < n && base.heap[i].k > base.heap[l].k) i = l;
        if (r < n && base.heap[i].k > base.heap[r].k) i = r;
        if (i == j) break;
        base = swap(base, i, j);
    }
    return base;
}

struct Queue InitQueue(int n)
{
    struct Queue q;
    struct Help *d = (struct Help*)malloc(n * sizeof(struct Help));
    q.heap = d;
    q.cap = n;
    q.count = 0;
    return q;
}

int QueueEmpty(struct Queue q)
{
    return (q.count == 0);
}

struct Queue Insert(struct Queue q, struct Help x)
{
    int i = q.count;
    if (i == q.cap) printf("FULL");
    q.count = i + 1;
    q.heap[i] = x;
    while (i > 0 && q.heap[(i - 1) / 2].k > q.heap[i].k) {
        q = swap(q, (i - 1) / 2, i);
        i = (i - 1) / 2;
    }
    return q;
}

struct Help ExtractMin(struct Queue q)
{
    if (q.count == 0) printf("NULL");
    return q.heap[0];
}

int main()
{
    int n, m, i, ans = 0, cur = 0, cluster = 0;
    struct Help x;
    scanf("%d\n", &n);
    scanf("%d\n", &m);
    int *data = (int*)malloc(2 * m * sizeof(int));
    for (i = 0; i < m; i++) {
        if (i != m - 1) {
            scanf("%d ", &data[2 * i]);
            scanf("%d\n", &data[2 * i + 1]);
        }
        else {
            scanf("%d ", &data[2 * i]);
            scanf("%d", &data[2 * i + 1]);
        }
    }
    struct Queue q = InitQueue(m);
    while (cur < 2 * m) {
        if (q.count > 0 && q.heap[0].k <= ans) {
            cluster--;
            q.count--;
            if (q.count > 0) {
                q.heap[0] = q.heap[q.count];
                q = heapify(0, q.count, q);
            }
        }
        if (data[cur] <= ans) {
            if (cluster < n) {
                cluster++;
                x.k = x.v = ans + data[cur + 1];
                q = Insert(q, x);
                cur += 2;
            }
        }
        ans++;
    }
    if (q.count > 0) ans = q.heap[q.count - 1].v;
    printf("%d\n", ans);
    free(data);
    free(q.heap);
    return 0;
}