ll@N,@K,A[2K],s=0,d=9d9,o=0,m,r=N+1,t;rep(i,K)A[i+K]=(A[i]=rd_ll())+2N;rep(i,K)if(d>A[i+1]-A[i])s=i,d=A[i+1]-A[i];while(o<r-1){m=(o+r)/2;d=2m;REP(j,A[s]+1,A[s+1],2){t=j;rep(i,1,K){if(A[s+i+1]<=t+d)t=3N;t=max(A[s+i]+1,t+d);}if(t+d<=j+2N)o=m;}o<m?r=m:r;}wt(2o);