C++ :

#include <stdio.h>
#include <string.h>
#include <vector>
#include <algorithm>
using namespace std;

vector<int> edge[105];
int dp[105][105];

int main()
{
    int i,j,n,m,x,y,k;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        for (i=0;i<n;i++)
        {
            edge[i].clear();
        }
        for (i=0;i<m;i++)
        {
            scanf("%d%d",&x,&y);
            x--;y--;
            edge[min(x,y)].push_back(max(x,y));
        }
        memset(dp,-1,sizeof(dp));
        dp[0][0]=1;
        for (i=0;i<n;i++)
        {
            for (j=0;j<n;j++)
            {
                if (dp[i][j]==-1) continue;
                if (i<=j)
                {
                    for (k=0;k<edge[i].size();k++)
                    {
                        int e=edge[i][k];
                        if (e==j && e!=n-1) continue;
                        dp[e][j]=max(dp[e][j],dp[i][j]+1);
                    }
                }
                else
                {
                    for (k=0;k<edge[j].size();k++)
                    {
                        int e=edge[j][k];
                        if (e==i && e!=n-1) continue;
                        dp[i][e]=max(dp[i][e],dp[i][j]+1);
                    }
                }
            }
        }
        if (dp[n-1][n-1]==-1) printf("1\n");
        else printf("%d\n",dp[n-1][n-1]-1);
    }
    return 0;
}