ll@h,@w;string@s[h],@t[h];
ll a=sum[ll][y,0,h,x,0,w](s[y][x]==t[y][x]);
ll b=sum[ll][y,0,h,x,0,w](s[y][x]==t[h+~y][w+~x]);
wt(a?b?-1:3.128936:b?3.531740:2.641632);