intrpolation search is not working with this series

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                                  This is the code
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#include<stdio.h>
#include<string.h>
#include<math.h>
struct bs
{
        int key;
        int l,h,m;
        int a[10];
};
typedef struct bs bs;
int bin_s(bs);
int main()
{
        int i;
        bs b;
        printf("enter the data\n");
        for(i=0; i<10;i++ )
                scanf("%d",&b.a[i]);
        printf("enter the Key world\n");
        scanf("%d",&b.key);
        b.l=0;
        b.h=9;
        b.m = bin_s(b);
        printf("data found:= %d\n",b.m);
        return -1;
}
int bin_s(bs b)
{
        if(b.h > b.l)
        {
                b.m = b.l + ((b.h - b.l) / (b.a[b.h] - b.a[b.l])) * (b.key - b.a[b.l]);
                sleep(1);
                printf("b.m:= %d\n",b.m);
                if(b.key == b.a[b.m])
                {
                        return b.m;
                }
                else if(b.a[b.m] < b.key)
                {
                        b.l=b.m + 1;
                        b.m = bin_s(b);
                        return b.m;
                }
                else
                {
                        b.h= b.m - 1;
                        b.m = bin_s(b);
                        return b.m;
                }

        }
        printf("key not found\n");
        return -1;
}

And the series:=
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                                        series
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enter the data
1
3
5
7
9
11
15
16
17
18
enter the key value
12