#include <stdint.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>

uint16_t graphAsArray[20002][20002];
uint16_t graphNbEdges[20002];
uint16_t path[20002];

int nbPeople;

uint16_t getEdgeIndex(uint16_t src, uint16_t dst) {
    for (uint16_t i = 0; i < graphNbEdges[src]; i++) {
        if (graphAsArray[src][i] == dst) {
            return i;
        }
    }
    return (uint16_t)-1;
}

void addEdge(uint16_t src, uint16_t dst) {
    uint16_t index = graphNbEdges[src]++;
    graphAsArray[src][index] = dst;
}

void removeEdge(uint16_t src, uint16_t dst) {
    uint16_t index = getEdgeIndex(src, dst);
    uint16_t replace = --graphNbEdges[src];
    graphAsArray[src][index] = graphAsArray[src][replace];
}

bool searchPath(uint16_t source, uint16_t target) {
    if (source == target)
        return true;
    
    for (int i = 0; i < graphNbEdges[source]; i++) {
        uint16_t dst = graphAsArray[source][i];
        if (path[dst] == (uint16_t)-1) {
            path[dst] = source;

            if (searchPath(dst, target))
                return true;
        }
    }

    return false;
}

int fordFulkerson(uint16_t source, uint16_t target) {
    uint16_t u, v;
    int maxFlow = 0;

    for (;;) {
        memset(path, -1, sizeof(path));
        if (!searchPath(source, target))
            break;

        v = target;
        while (v != source) {
            u = path[v];
            removeEdge(u, v);
            addEdge(v, u);
            v = path[v];
        }

        maxFlow++;
    }

    return maxFlow;
}

int main(void) {
    int nbDeclarations;
    int person, preference;
    scanf("%d %d", &nbPeople, &nbDeclarations);

    for (int i = 0; i < nbDeclarations; i++) {
        scanf("%d %d", &person, &preference);
        addEdge(person*2, preference*2+1);
    }

    for (int i = 0; i < nbPeople; i++) {
        addEdge(nbPeople*2, i*2);
        addEdge(i*2+1, nbPeople*2+1);
    }

    int result = fordFulkerson(nbPeople*2, nbPeople*2+1);
    if (result == nbPeople) {
        printf("YES\n");
    } else {
        printf("NO\n");
    }
}